Tidy up documentation

[kytta]

Short description

This PR regards issue #532. It doesn't introduce much new documentation, but rather re-arranges the existing information into more granular pages.

Proposed changes

• re-arrange documentation roughly based on the Diataxis framework
  • add API docs for the CreatePDF() function
    • it might be a better idea to document and type-hint public methods in-code and use autodoc
• add sphinx_reredirects to redirect people from the old pages to the new ones
  • it might be a better idea to use RTD's page redirects, but I don't have access to the admin panel
• expanded definitions of intersphinx to include Django docs
  • also added a small Sphinx plugin to resolve the :django: refs

Preview: https://xhtml2pdf--729.org.readthedocs.build/en/729/quickstart.html