Render documentation using Sphinx

[bartfeenstra]

This fixes https://github.com/bartfeenstra/betty/issues/636

[codecov[bot]]

Codecov Report

Merging #637 (dd0cab0) into master (150f8d1) will decrease coverage by 0.15%. The diff coverage is 87.14%.

@@            Coverage Diff             @@
##           master     #637      +/-   ##
==========================================
- Coverage   96.90%   96.74%   -0.16%     
==========================================
  Files          46       47       +1     
  Lines        3937     3995      +58     
==========================================
+ Hits         3815     3865      +50     
- Misses        122      130       +8     

Impacted Files	Coverage Δ
betty/error.py	100.00% <ø> (ø)
betty/extension/__init__.py	100.00% <ø> (ø)
betty/extension/nginx/__init__.py	89.36% <ø> (ø)
betty/graph.py	100.00% <ø> (ø)
betty/jinja2.py	96.78% <ø> (ø)
betty/locale.py	96.65% <ø> (ø)
betty/serve.py	78.94% <ø> (ø)
betty/documentation.py	80.95% <80.95%> (ø)
betty/cli.py	91.71% <85.18%> (-1.47%)	:arrow_down:
betty/about.py	94.44% <93.33%> (+8.73%)	:arrow_up:
... and 5 more

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[bartfeenstra]

Closing in favor of https://github.com/bartfeenstra/betty/pull/1143