docs: add RDF4J Store and Client user docs

[edmondchuc]

Summary of changes

This PR adds user docs for the new RDF4J Store and Client that was introduced in PR #3316.

See the docs preview here: https://rdflib--3319.org.readthedocs.build/en/3319/rdf4j/

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.

[nicholascar]

A really fantastic documentation effort here @edmondchuc! I read through it and can understand all the things you've implemented in the Store and the Client. I only wish all RDFLib documentation was produced to such an excellent standard.