[solidago] init docs content and use mkdocs to build docs pages

[amatissart]

Description

Initialize documentation website for Solidago.
The current state is visible on https://solidago.tournesol.app/

Using:

• mkdocs: https://www.mkdocs.org/
• mkdocs-material: https://squidfunk.github.io/mkdocs-material/
• mkdocstrings: https://mkdocstrings.github.io/

Checklist

• I added the related issue(s) id in the related issues section (if any)
  • if not, delete the related issues section
• I described my changes and my decisions in the PR description
• I read the development guidelines of the CONTRIBUTING.md
• The tests pass and have been updated if relevant
• The code quality check pass

[amatissart]

I configured a CNAME DNS record in order to deploy the docs to a custom with Github Pages. For now it needs to be pushed manually with mkdocs gh-deploy but that could also be included in the CI workflows.

The current state is visible on https://solidago.tournesol.app

It's still missing quite a lot of docstrings, it needs a little more polish here and there, and more examples would also be helpful.

@lenhoanglnh What is your opinion about the structure of the docs? Are there specific sections to create in order to help onboard researchers?