This PR implements a new docs site, based on fumadocs.

I purposefully did not just update the existing docs site, so that we can compare the changes, and remove any cruft that may be left over.

So far, I've done the following:

• Implemented the docs and examples content pages (with slight modifications to the content since titles are now generated from the frontmatter of the mdx files)
• Moved over the pricing page (a one-off page that was in React)
• Moved over the thank you page (an mdx page)
• Moved over the sign in/sign up pages and functionality and put the sign in button in the header
• Added an llms.txt and llms-full.txt file which are automatically generated from the docs content (useful for LLMs to understand out APIs)

What's left:

• Move over the other one-off pages (about, legal, etc.)
• Migrate the home page to the new home page and rewrite to pure React instead of mdx
• Update MDX components
  • The examples component is a bit of a mess (see image below)
    • likely needs some better scoped styles to make sure that github and stackblitz links do not turn purple
    • The example background feels broken since it is just embedded in the page
  • For the examples page, let's recreate our examples grid https://www.blocknotejs.org/examples using fumadocs' card grid: https://fumadocs.dev/docs/ui/markdown#cards
• The whole bottom footer is missing https://fumadocs.dev/docs/ui/layouts/page#footer
• Do we want the edit on github button? https://fumadocs.dev/docs/ui/layouts/page#edit-on-github or the feedback button?