Restructure the documentation and render docstrings

This MR exclusively deals with declearn's documentation, and paves the way towards publishing it online using GitLab pages (or any other documentation-hosting tool).

It performs the following changes:

• Restructure the increasingly-long README file into an ensemble of pages under the newly-created "docs" folder, and shrink the README into a minimal file.
• Automate the copy of content from the README to the welcome page of the "docs" folder.
• Set up tools to build the documentation into a static website, and automatically generate the API reference part of the docs.
• Marginally restructure the code's documentation to improve its rendering by mkdocs/mkdocstrings-python.
• Write up a small reference guide on docstrings style.
• Marginally change extra dependencies' specifications:
  • "all" still covers all non-developer extra dependencies
  • "tests" now only covers code-testing tools (enabling to run tests on incomplete installations)
  • "docs" was added, to cover docs-building tools (only)

At the moment, the following points are left untackled:

• Set up the website's building and publishing: this will be done on a separate repository.
• Tweak the rendering options of the website to make it nicer ~ more custom. This is delayed to future, possibly incremental, efforts.