Improve autodoc docs

[timhoffm]

• Clarify docs on how to reference directive options
• autodoc improvements:
  • Cross-link the 'autodoc_default_options' to the descriptions of the respective directive options
  • Add types and defaults to config parameters
    Please check how you would like the type and default to be soecified. - It's not consistent throughout
    the codebase, so I took some convention
  • deleted the docs for the config value autodoc_default_flags, which already has been removed a long time ago.
  • minor formatting improvements

[AA-Turner]

• Please check how you would like the type and default to be soecified. - It's not consistent throughout the codebase, so I took some convention

I spent several weeks over the summer rewriting usage/configuration.rst, so that style is preferred: https://www.sphinx-doc.org/en/master/usage/configuration.html

A