[WIP] Adopt new docspec and integrate with Novella

[NiklasRosenstein]

Goal of this PR is to

• Adopt to new docspec 2.x API that is currently WIP
• Integrate with Novella
  • Implement a basic version of the @pydoc tag
  • Update SinglePageRenderer interface (or add a new interface) for rendering single docspec objects (instead of only a module)
  • Make a resolver separately configurable (as it is not provided through SinglePageRenderer and it wouldn't make much sense for it to have such a capability)
  • Make MarkdownRenderer produce Novella @anchor tags
  • Make CrossRefProcessor procuce Novella {@link} tags
  • (Option to) hide the module name when referencing it with the @pydoc tag? (If yes, maybe we should include the module name somewhere else in the generated documentation for the module members)
  • Automatically detect source linker configuration if possible
  • Add an option to use anchor HTML tags instead of Markdown header IDs for links between API objects (maybe enabled by default) because the IDs for Markdown headers might not be unique (e.g. two API objects with the same name in the same page)
• Make codebase fully typed
• Update Pydoc-Markdown's own documentation to be built with Novella (the latest can be found on GitHub pages: https://niklasrosenstein.github.io/pydoc-markdown/)

Late state considerations:

• Work on making Novella Python 3.7 3.8-3.9 compatible? (Currently it is only compatible with 3.10)

If you are interested in an introduction to how the new system works, check out the Using Novella guide in the documentation.