Implement llms.txt standard

As part of the NumFocus SDG, we want to implement the `llms.txt` standard and generate markdown docs to be served alongside the documentation.

This will follow the pattern established in https://github.com/panel-extensions/panel-material-ui/pull/663.

## Steps

1. Build Markdown versions of the relevant parts of the docs we want the LLMs to consume
2. Create the `llms.txt` file
3. Add instructions in the [holoviz-skills](https://github.com/holoviz-dev/holoviz-skills) repo that points to the `llm.txt` files.

This way, the AI models will always have easier access to the most up-to-date and relevant parts of our documentation.