Add Markdown formatter + exporters

[josevalim]

There has been a lot of changes around this area, both in ExDoc and in AI, and I have been postponing this.

Perhaps more interesting, the following has happened:

1. We have improved some of ExDoc so there is less duplication across formatters
2. Other documentation tools are now including "copy markdown" or "view markdown" functionality on their pages, as seen here: https://modelcontextprotocol.io/introduction
3. Some LLM tools are now including a llms.txt that is basically an index and points to other files, as generating a single llms.txt may blow up the context

TODOs:

• Introduce a Markdown formatter which runs by default
• Add a "copy markdown" button to the top of all pages if both HTML and Markdown are enabled
• Move manifest management outside of the formatter
• Remove duplication across formatters

[lud-wj]

Hello,

I agree with your 3 points :)

I really like how bitsui is doing it. Basically you can append /llms.txt to any doc page and get a markdown version.

I think in hexdocs we could have the same, replacing .html by .llms.txt for instance. Honestly it would even be nice to be able to do so at a function level (via an option to the formatter) if we want an LLM to retrieve docs for a function, as some modules are huge.