[docs-infra] Create llms.txt

[Janpot]

• render react docs to markdown
• serve under /<react-docs-path>.md. e.g. for /react/components/accordion add /react/components/accordion.md
• add a /llms.txt
• add a link to the markdown version on each page (I guess this is temporary, we can replace with "Ask AI" dorpdown?)
  
• currently the markdown is generated on the fly, right when the docs are built. We could also opt for pregenerating them and check them in our repo, this would stronger guarantee correctness in the absence of tests.

How does the mdx to md transformation currently work:

• parse mdx files into mdx ast
• statically analyze the jsx parts and render reference tables and demos, extract out the description and title
• convert those demos and reference tables jsx AST into md AST.
• error on unrecognized jsx
• run the mdast stringify plugin to obtain markdown

Drawbacks

• It only statically analyzes the JSX, this restricts a lot the valid jsx we are able to write
• Introducing new syntax and jsx elements will be cumbersome

Alternatives that are more maintainable:

• compile the mdx and render with react, use custom react reconciler to render the react tree to markdown instead of html.
• render the docs to html and convert the html back to markdown. e.g. with https://www.npmjs.com/package/turndown

Will likely move to one of those methods as to not disrupt the velocity of the project

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