Update assistant entry points

[ethanpalm]

Documentation changes

This PR documents the new assistant entry point of highlighting text snippets, adds the missing ask AI on code blocks, and reformats the list of entry points into H3 sections.

Closes https://linear.app/mintlify/issue/DOC-211/document-new-ask-ai-entry-points

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• [ ] Code examples work as written
• [ ] Commands and configurations are correct
• [ ] Links resolve to the right destinations
• [ ] Prerequisites and requirements are accurate

✅ Clarity and completeness

• [ ] Instructions are clear and easy to follow
• [ ] Steps are in logical order
• [ ] Nothing important is missing
• [ ] Examples help illustrate the concepts

✅ User experience

• [ ] A new user could follow these docs successfully
• [ ] Common gotchas or edge cases are addressed
• [ ] Error messages or troubleshooting guidance is helpful

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[!NOTE] Restructures assistant usage docs with new entry points (highlight text, code block Ask AI, placement) and updates the model reference to Claude Opus 4.5.

• Docs (assistant):
  • Usage: Rewrites "Using the assistant" into structured subsections with visuals.
    • Adds UI placement options (search-bar button vs bottom bar) with side-by-side images.
    • Adds new entry points: highlight text (Add to assistant), code blocks (Ask AI), plus keyboard shortcut and URL param.
  • About: Updates model reference to Claude Opus 4.5.
  • Minor copy tweaks (e.g., heading to Configure the assistant).

^{Written by Cursor Bugbot for commit 5eb1a07f58ed962ad493b5b4131be7607d39fad5. This will update automatically on new commits. Configure here.}