docs: add explanation of documentation system architecture

[chrisburr]

Summary

• Add new explanation page documenting the DiracX documentation system
• Covers the Divio documentation system (tutorials, how-to, reference, explanation)
• Explains multi-repository architecture with mkdocs-diracx-plugin
• Documents three target audiences (users, administrators, developers)
• Describes navigation structure and edit URLs

Note: The edit URL fix for multi-repo documentation is being addressed in a separate PR to the mkdocs-diracx-plugin repository: DIRACGrid/mkdocs-diracx-plugin#5

Part of #614

[read-the-docs-community]

Documentation build overview

📚 diracx | 🛠️ Build #31188751 | 📁 Comparing 60b1080 against latest (a0b3053)

🔍 Preview build

Show files changed (117 files in total): 📝 116 modified | ➕ 1 added | ➖ 0 deleted

File	Status
404.html	📝 modified
index.html	📝 modified
REFERENCE/index.html	📝 modified
RUN_PROD/index.html	📝 modified
SECURITY/index.html	📝 modified
SSO/index.html	📝 modified
admin/index.html	📝 modified
dev/index.html	📝 modified
roadmap/index.html	📝 modified
user/index.html	📝 modified
admin/deploy_instance/index.html	📝 modified
admin/explanations/index.html	📝 modified
admin/how-to/index.html	📝 modified
admin/manage_dependencies/index.html	📝 modified
admin/manage_release/index.html	📝 modified
admin/reference/index.html	📝 modified
admin/tutorials/index.html	📝 modified
dev/explanations/index.html	📝 modified
dev/how-to/index.html	📝 modified
dev/manage_extension/index.html	📝 modified
dev/reference/index.html	📝 modified
dev/setup_environment/index.html	📝 modified
dev/tutorials/index.html	📝 modified
dev/web-arch/index.html	📝 modified
developer/contribute/index.html	📝 modified
developer/manage_extension/index.html	📝 modified
developer/setup_environment/index.html	📝 modified
user/explanations/index.html	📝 modified
user/how-to/index.html	📝 modified
user/reference/index.html	📝 modified
user/tutorials/index.html	📝 modified
user/web/index.html	📝 modified
admin/explanations/auth-with-diracx/index.html	📝 modified
admin/explanations/auth-with-external/index.html	📝 modified
admin/explanations/chart-structure/index.html	📝 modified
admin/explanations/configuration/index.html	📝 modified
admin/explanations/database-management/index.html	📝 modified
admin/explanations/opentelemetry/index.html	📝 modified
admin/explanations/sandbox-store/index.html	📝 modified
admin/explanations/user-management/index.html	📝 modified
admin/how-to/debugging/index.html	📝 modified
admin/how-to/install/index.html	📝 modified
admin/how-to/rotate-a-secret/index.html	📝 modified
admin/how-to/upgrading/index.html	📝 modified
admin/reference/env-variables/index.html	📝 modified
admin/reference/security_model/index.html	📝 modified
admin/reference/settings-and-preferences/index.html	📝 modified
admin/reference/values/index.html	📝 modified
admin/tutorials/authentication/index.html	📝 modified
admin/tutorials/run_locally/index.html	📝 modified
dev/explanations/components/index.html	📝 modified
dev/explanations/designing-functionality/index.html	📝 modified
dev/explanations/documentation-system/index.html	➕ added
dev/explanations/extensions/index.html	📝 modified
dev/explanations/run_demo/index.html	📝 modified
dev/explanations/testing/index.html	📝 modified
dev/how-to/add-a-cli-command/index.html	📝 modified
dev/how-to/add-a-db/index.html	📝 modified
dev/how-to/add-a-route/index.html	📝 modified
dev/how-to/add-a-setting/index.html	📝 modified
dev/how-to/add-a-task/index.html	📝 modified
dev/how-to/add-a-test/index.html	📝 modified
dev/how-to/add-functionality/index.html	📝 modified
dev/how-to/client-customization/index.html	📝 modified
dev/how-to/client-extension/index.html	📝 modified
dev/how-to/client-generation/index.html	📝 modified
dev/how-to/contribute/index.html	📝 modified
dev/how-to/create_application/index.html	📝 modified
dev/how-to/develop-legacy-dirac/index.html	📝 modified
dev/how-to/extend-diracx/index.html	📝 modified
dev/how-to/use-the-demo/index.html	📝 modified
dev/reference/application-state/index.html	📝 modified
dev/reference/client-metapathfinder/index.html	📝 modified
dev/reference/coding-conventions/index.html	📝 modified
dev/reference/configuration/index.html	📝 modified
dev/reference/db-transaction-model/index.html	📝 modified
dev/reference/dependency-injection/index.html	📝 modified
dev/reference/entrypoints/index.html	📝 modified
dev/reference/env-variables/index.html	📝 modified
dev/reference/pixi-tasks/index.html	📝 modified
dev/reference/security-policies/index.html	📝 modified
dev/reference/security-properties/index.html	📝 modified
dev/reference/test-recipes/index.html	📝 modified
dev/reference/writing-tests/index.html	📝 modified
dev/tutorials/advanced-tutorial/index.html	📝 modified
dev/tutorials/develop-web/index.html	📝 modified
dev/tutorials/getting-started/index.html	📝 modified
dev/tutorials/larger-developments/index.html	📝 modified
dev/tutorials/making-changes/index.html	📝 modified
dev/tutorials/play-with-auth/index.html	📝 modified
dev/tutorials/run-locally/index.html	📝 modified
dev/tutorials/write-docs/index.html	📝 modified
user/reference/client-configuration/index.html	📝 modified
user/reference/known-installations/index.html	📝 modified
user/reference/programmatic-usage/index.html	📝 modified
user/tutorials/getting-started/index.html	📝 modified
user/web/list_and_share_applications/index.html	📝 modified
user/web/login_out/index.html	📝 modified
user/web/monitor_jobs/index.html	📝 modified
admin/how-to/install/connect/index.html	📝 modified
admin/how-to/install/convert-cs/index.html	📝 modified
admin/how-to/install/embracing/index.html	📝 modified
admin/how-to/install/install-kubernetes/index.html	📝 modified
admin/how-to/install/installing/index.html	📝 modified
admin/how-to/install/minimal-requirements/index.html	📝 modified
admin/how-to/install/register-a-vo/index.html	📝 modified
admin/how-to/install/register-the-admin-vo/index.html	📝 modified
dev/explanations/components/api/index.html	📝 modified
dev/explanations/components/cli/index.html	📝 modified
dev/explanations/components/client/index.html	📝 modified
dev/explanations/components/db/index.html	📝 modified
dev/explanations/components/routes/index.html	📝 modified
dev/how-to/use-the-demo/swagger/index.html	📝 modified
dev/how-to/use-the-demo/web/index.html	📝 modified
user/reference/programmatic-usage/command-line-interface/index.html	📝 modified
user/reference/programmatic-usage/https-interface/index.html	📝 modified
user/reference/programmatic-usage/python-interface/index.html	📝 modified