docs: update documentation structure

[adohe]

Summary:

• reorganize the documentation navigation into the current visible sections
• add collapsible sidebar groups and refine sidebar scrollbar/search/menu styling
• add focused getting started, concepts, cookbook, reference, and troubleshooting pages

Verification:

• .venv/bin/sphinx-build -E -a -b html docs /private/tmp/areno-docs-preview

[gemini-code-assist]

Code Review

This pull request significantly updates the documentation structure for the AReno project, introducing new conceptual guides, cookbooks, and a troubleshooting section, while also adding a new collapsible sidebar feature for improved navigation. The code review provides a valuable improvement to the sidebar functionality, suggesting that sections should default to collapsed unless they contain the active page, which I have included in the feedback summary.

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[xsuler]

/lgtm