Documentation rework that plugins usage and modules are crosslinked

[WhiteGobo]

This is work in correspondence to my issue #3307
This targets better findability of plugins, here only with more documentation links and better descriptions.

current documentation status

The plugin documentation links to base modules, that contain builtin plugins. The plugin modules dont have any more detailed description there. But in general they list their submodules and each submodule equals one plugin.

Methods, that use the plugins generally link only to some example plugins. graph._init__; graph.serialize; graph.parse; Result.serialize.

The general rdflib.plugin module isnt connected to any specific plugin module.

goal of pr

There should be a link between methods and their used specific plugin modules and vice versa.
I want to link all plugin-able thingies to the general plugin module rdflib.plugin:

• graph.parse with plugins.parsers;
• graph.serialize with plugins.serializers`;
• Result.serialize with plugins.sparql.results`;
• graph.__init__ with plugins.stores`
• everything to rdflib.plugin

Also i added a base description for Serilalizer.serialize

Summary of changes

Reworked description for all modules, that are linked as plugin description.
Added crosslink for plugin related things. linked. See list in goals.
Reworked description of Graph.__init__

I added a base description for Serilalizer.serialize

No change outside from documentation.

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. See Better documentation for plugins, like parser and serializer #3307
• If the change has a potential impact on users of this project:
  • 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]

Wonderful to have a documentation PR! Thanks @WhiteGobo