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

[weronikaolejniczak]

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) 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: [POC] feat(website): configure llms plugin #8827)

Acceptance criteria

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