docusaurus-plugin-llms-txt : Take into account the `Accept` HTTP header

[Nicolapps]

Some agents like Claude Code include the following HTTP header when fetching documentation:

Accept: text/markdown, text/html, */*

When an agent tries to fetch a doc page using that header, it would be great to serve the Markdown version automatically. This behavior is already implemented by some doc websites such as the Vercel docs and the Claude Code docs.

[Devon-White]

We handle this via a nginx config, could you do something similar? I can certainly look into this being a config option when io have some free time :)