docs: Move to `gp-libs` (our internal helpers for sphinx)

[tony]

Changes

• Render changelog in linkify_issues
• Fix Table of contents rendering with sphinx autodoc with sphinx_toctree_autodoc_fix

• Deprecate sphinx-autoapi, per above fixing the table of contents issue

  This also removes the need to workaround autoapi bugs.

• Test doctests in our docs via pytest_doctest_docutils (built on doctest_docutils)

[codecov[bot]]

Codecov Report

Merging #265 (15a59ea) into master (6ac525c) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
- Coverage   93.66%   93.65%   -0.01%     
==========================================
  Files          11       11              
  Lines        1042     1041       -1     
==========================================
- Hits          976      975       -1     
  Misses         66       66              

Impacted Files	Coverage Δ
docs/conf.py	58.13% <100.00%> (-0.49%)	:arrow_down:

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