Add markdown action buttons reference page

[sbishel]

Summary

Documents the new markdown action buttons interactive surface, where an
inline post-markdown link using mmaction://<id> plus a matching
mm_blocks_actions post prop dispatches a post action to the
integration's endpoint — an alternative to message-attachment buttons.

This is the developer-facing reference for the feature; framed as one
binding surface in a broader Interactive Messages framework so it can
sit alongside additional surfaces / action types without needing a
rewrite.

Changes

• New: site/content/integrate/reference/markdown-actions/_index.md
  — canonical reference page (weight 45, slotted between
  message-attachments and message-priority). Covers use cases,
  example payload, REST + plugin submission examples, mmaction://
  link syntax, mm_blocks_actions schema and types, click-dispatch
  flow, callback shape, validation limits, error reference, and
  security considerations.
• Modified: site/content/integrate/plugins/interactive-messages/_index.md
  — added a short "Markdown action buttons" section near the top
  cross-linking to the canonical reference so plugin authors discover
  the new surface from the existing interactive-messages page.

Ticket Link

Fixes https://mattermost.atlassian.net/browse/MM-68387

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 5628d52

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 26709435-aa48-45d9-896d-543d82d3ef4e

📥 Commits

Reviewing files that changed from the base of the PR and between 835f947 and 5628d52.

📒 Files selected for processing (2)

• site/content/integrate/plugins/interactive-messages/_index.md
• site/content/integrate/reference/markdown-actions/_index.md

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📝 Walkthrough

Walkthrough

This PR adds comprehensive documentation for the Mattermost markdown action buttons feature. It includes an overview subsection in the interactive messages guide and a complete reference page covering syntax, registry structure, payload examples, validation limits, error codes, and security considerations.

Changes

Markdown Action Buttons Documentation

Layer / File(s)	Summary
Interactive Messages overview update site/content/integrate/plugins/interactive-messages/_index.md	Added a brief subsection explaining that markdown action buttons can be embedded directly in post content using mmaction:// links backed by mm_blocks_actions post properties.
Reference page introduction and conceptual overview site/content/integrate/reference/markdown-actions/_index.md	Front matter and introductory sections defining when markdown action buttons are useful, contrasting them with attachment-based actions, and explaining the two required components and overall client/server behavior.
Usage examples and implementation guides site/content/integrate/reference/markdown-actions/_index.md	Complete post payload examples and practical code snippets for submitting markdown action buttons via REST API (curl) and plugin APIs (Go), demonstrating both creation and update workflows.
Technical specification and dispatch flow site/content/integrate/reference/markdown-actions/_index.md	Detailed documentation of post-update validation, mmaction:// link syntax rules, mm_blocks_actions registry field definitions with query merge semantics, supported action types, and the step-by-step click dispatch lifecycle through to integration callback handling.
Validation limits, errors, security, and references site/content/integrate/reference/markdown-actions/_index.md	Creation and update validation limits (registry entry count, field lengths, query constraints), error reference with specific error IDs, security considerations for server-to-server invocation and input validation, and cross-references to related features.

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Add markdown action buttons reference page' accurately summarizes the primary change—adding a new reference documentation page for markdown action buttons.
Description check	✅ Passed	The description is directly related to the changeset, providing clear context about the markdown action buttons feature, the new reference page, and the modification to the interactive-messages page.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch markdown_actions

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[larkox]

The docs feel a bit too verbose, but that may be a personal preference. Apart of that, LGTM.

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 5534512