docs: change structure

[paulfantom]

What is missing?

The current README.md file is a bit too large to navigate efficiently. Plus we hide some of the documents in docs/ and some we embed in README.md.

Why do we need it?

Improving UX.

Environment

• kube-prometheus version: all

Anything else we need to know?:

I propose to split README.md into:

• CONTRIBUTING.md - to satisfy GitHub guidelines
• docs/<top_level_header_from_readme.md>.md

In README.md we should leave general information about the project, migration warnings, TOC, License link, and any other short information that would be required by a user. Everything else could be put into separate files and linked from README.

Later we can combine this with prometheus-operator docs into one docs site. We may use some tools like https://antora.org/ or https://github.com/snipem/monako to do this efficiently.

@prometheus-operator/prometheus-operator-reviewers wdyt?

[brancz]

Yes please.