[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?

[amatissart]

I think this PR is ready to be merged now. The docs are visible on https://solidago.tournesol.app/

@GresilleSiffle @lenhoanglnh Do you want to have a look before I merge?

[GresilleSiffle]

It looks very good to me @amatissart 👌

Feel free to merge when you want, maybe after a review from @lenhoanglnh

In the future the Get Started page could benefit from more paragraphs, to help the users understand what are the goals of Solidago and in which context the library is relevant. If I understood correctly, this PR is the initial set-up of the documentation, not an effort to document everything exhaustively right?

Very minor for now: some functions use a prescriptive form of docstring, as recommended by PEP257 (like "Standardize scores..."), whereas other use a descriptive form (like "Aggregates the scores..."). We could decide which form we want to use in the doc.

[GresilleSiffle]

An idea: we could add a link to the documentation in the footer of the front end, under the section Research.

[amatissart]

An idea: we could add a link to the documentation in the footer of the front end, under the section Research.

Done in 5d93da4