Migrate to mkdocs

[vemonet]

Summary of changes

Hi @nicholascar @ajnelson-nist, as discussed in #3128 I migrated the documentation from rst format with sphinx, to markdown with mkdocs (material for mkdocs)

• Migrated all .rst pages to .md. I merged the 5 plugins pages in 1 (all pages were small ~50 lines, it's easier to maintain and read to have them all in one page, without being overwhelming), we can also put it back as 5 different pages if required.

• Migrated all docstrings to markdown with google style, generated API references docs using mkdocstring e.g.

  def _make_redirect_request(request: Request, http_error: HTTPError) -> Request:
      """Create a new request object for a redirected request.
  
      The logic is based on [HTTPRedirectHandler](https://github.com/python/cpython/blob/b58bc8c2a9a316891a5ea1a0487aebfc86c2793a/Lib/urllib/request.py#L641-L751) from urllib.request.
      
      Example:
          ```python
          >>> req = _make_redirect_request(request, http_error)
  
          ```
  
      Args:
          request: The original request that resulted in the redirect.
          http_error: The response to the original request.
  
      Returns:
          A new request object to the location indicated by the response.
  
      Raises:
          HTTPError: the supplied `http_error` if the redirect request cannot be created.
          ValueError: If the response code is None.
      """

• All doctests are working and passing, they just required to add an extra newline before the ending ```

• I have previously mentioned I was planning to look into alternatives for running markdown codeblocks, e.g. usingpytest-markdown-docs. Introducing tests on the markdown codeblocks would require a lot of changes to the documentation, there are already enough.

• I chose indigo for the primary color because it is one of the few colors that constrats well with the green and yellow of the logo, and it is close to the background of the original semantic web logo

• Website splits in 3 top nav: Usage, API Reference, Development. With the pages in the current section listed on the left

• Updated the .readthedocs.yaml config according to their docs, removed sphinx from dependencies and updated most build scripts

You can easily start it locally with:

poetry install
poetry run mkdocs serve

I have included screenshots of the website, let me know if there is anything that needs to be done

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement.
  • Considered adding an example in ./examples.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.