docs: fix API docs

[uniqueg]

Description

• Move docs dependencies from docs/api to repository root
• Define optional dependencies docs and dev in setup.py, allowing users to do, e.g., this:

  pip install foca[dev,docs]

• Configure .readthedocs.yaml to install FOCA package with docs and dev dependencies (as optional dependencies, see above) during API docs build (previously did not install FOCA at all, only the docs dependencies)
• Fix file path reference and remove dummy exclude pattern in API docs configuration docs/api/conf.py
• Remove missing section index from docs/api/index.rst
• Fix several style-related Sphinx warnings
• Remaining "duplicate object description" warnings seem to arise from Attributes definitions in Enum and pydantic model class definitions (perhaps similar to this issue; however, they seem not to interfere with the doc building process and the resulting docs look/behave as expected

Fixes #207

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation (or documentation does not need to be updated)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not reduced the existing code coverage

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (f03506d) to head (8ebca3e).

Additional details and impacted files

@@            Coverage Diff            @@
##               dev      #208   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines          975       975           
=========================================
  Hits           975       975           

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