docs: Fix llms.txt, open graph, and shadcn

[devongovett]

Fixes a few bugs in the docs:

1. URLs in llms.txt have react-aria/ and s2/ as prefixes, but we don't need this since they are on separate domains now.
2. Open graph images had the wrong url in production due to extra slash.
3. Step counters on the getting started page were clipped.
4. Shadcn registry needs to be flattened as per Add React Aria registry shadcn-ui/ui#9121. Instead of installing as @react-aria/tailwind/ListBox it is now @react-aria/tailwind-listbox. Also generates a root registry.json file listing all of the available components, and uses the @react-aria namespace in production builds (full URL in other environments).

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs