[Docs] Include llms.txt and .md versions of the documentation pages

The purpose of this task is to make our documentation website LLM-friendly. By that I mean, we can include an `llms.txt` (see the [specification](https://llmstxt.org/)) and an `.md` version of all pages, e.g.:
`https://eui.elastic.co/docs/components/navigation/buttons/button/` and `https://eui.elastic.co/docs/components/navigation/buttons/button.md` with an LLM-driven frontmatter.

## Options

- We can test https://github.com/rachfop/docusaurus-plugin-llms (see the PoC: https://github.com/elastic/eui/pull/8827)

## Acceptance criteria

- `llms.txt` should be created and exposed on our documentation website
- each LLM-relevant page should have an `.md` version