Documentation changes

This PR adds an "AI optimization" group and splits the https://mintlify.com/docs/ai-ingestion page into multiple articles.

• llms.txt: Overview of what llms.txt are, why they're useful, and how to access them for your site
• Contextual menu: How to use the menu. Updates the custom example from Perplexity to Gemini since Perplexity is an option in the menu
• Markdown export: Covers .md method and keyboard shortcut for getting Markdown versions of pages

This PR also

• Moves other AI-related pages to the new group
• Added PreviewButton snippet since it's used in several pages

Closes https://linear.app/mintlify/issue/DOC-82/llmstxt-dedicated-docs-page-for-better-search
Closes https://linear.app/mintlify/issue/DOC-91/ia-for-ai-docs
Closes https://linear.app/mintlify/issue/DOC-88/update-ai-docs

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