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

[amatissart]

Description

Initialize documentation website for Solidago

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