feat: add export or view page as MD

[harsh62]

Description of changes:

Summary

• Add structured markdown export (llms.txt) for AI coding assistants, following the emerging https://llmstxt.org/
• Generate llms.txt index, llms-full.txt concatenated export, and individual per-page .md files for all ~285 Gen2 doc pages
• Add "Use with AI" dropdown button on Gen2 content pages with options to copy page markdown to clipboard or open it in a new tab

Details

Build script (tasks/generate-llms-txt.mjs):

• Walks directory.json to collect all Gen2 pages (routes under /[platform]/)
• Transforms MDX to clean markdown: strips exports/imports, converts InlineFilter → HTML comments, BlockSwitcher/Block → headings,
  Callout → blockquotes, Accordion →
  Details

  , strips Overview/Video/YoutubeEmbed/JSX comments
• Protects fenced code blocks from regex transforms using placeholder extraction
• Resolves by reading and inlining referenced fragment MDX files
• Outputs to public/ai/ — served by Next.js in both dev and production
• Runs as part of prebuild (after directory.json generation)

UI component (src/components/MarkdownMenu):

• "Use with AI" button with ✨ emoji and dropdown chevron, positioned right of page heading
• Two menu items: "Copy page MD" (clipboard) and "Open this page as MD" (new tab)
• Hidden on Gen1 pages, home page, and overview/index pages
• Guards against copying HTML if markdown file is unavailable

Instructions

If this PR should not be merged upon approval for any reason, please submit as a DRAFT

Which product(s) are affected by this PR (if applicable)?

• amplify-cli
• amplify-ui
• amplify-studio
• amplify-hosting
• amplify-libraries

Which platform(s) are affected by this PR (if applicable)?

• JS
• Swift
• Android
• Flutter
• React Native

Please add the product(s)/platform(s) affected to the PR title

Checks

• Does this PR conform to the styleguide?

• Does this PR include filetypes other than markdown or images? Please add or update unit tests accordingly.

• Are any files being deleted with this PR? If so, have the needed redirects been created?

• Are all links in MDX files using the MDX link syntax rather than HTML link syntax?
  
  ref: MDX: [link](https://docs.amplify.aws/)
  HTML: <a href="https://docs.amplify.aws/">link</a>

When this PR is ready to merge, please check the box below

• Ready to merge

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.