Role typings and documentation

[tony]

Changes

• docs(roles): Add typings to docs, split ToC
• docs(roles): Impove docstrings
• Rename GenericUrlRoleFn -> RoleFnReturnValue

[codecov]

Codecov Report

Merging #394 (8c48e06) into master (5dd3ebc) will increase coverage by 0.02%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
+ Coverage   66.55%   66.58%   +0.02%     
==========================================
  Files          43       43              
  Lines        1214     1215       +1     
==========================================
+ Hits          808      809       +1     
  Misses        406      406              

Files	Coverage Δ
src/django_docutils/lib/roles/email.py	50.00% <100.00%> (ø)
src/django_docutils/lib/roles/file.py	17.94% <ø> (ø)
src/django_docutils/lib/roles/github.py	35.71% <100.00%> (ø)
src/django_docutils/lib/roles/hackernews.py	50.00% <100.00%> (ø)
src/django_docutils/lib/roles/kbd.py	36.36% <ø> (ø)
src/django_docutils/lib/roles/leanpub.py	38.46% <100.00%> (ø)
src/django_docutils/lib/roles/pypi.py	50.00% <100.00%> (ø)
src/django_docutils/lib/roles/readthedocs.py	38.46% <100.00%> (ø)
src/django_docutils/lib/roles/twitter.py	41.66% <100.00%> (ø)
src/django_docutils/lib/roles/types.py	81.81% <100.00%> (+1.81%)	⬆️
... and 3 more

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