[#5261] generate reference documentation from docstrings

[MichaelAkvo]

TODO / Done

Summarize what has been changed / what has to be done in order to finalize the PR.

• [x] Use sphinx for reference documentation generation
• [x] Use github action to generate documentation and publish to http://akvo.github.io/akvo-rsr/

The choice for sphinx was due to its capability of recursively handling modules. Other options like portray, pdoc3, and mkdocs do not offer this possibility without significant effort (writing a custom handler). The result is not splendid (navigation is not intuitive), but it is usable and it can at least be expanded further.

Test plan

What tests are necessary to ensure this works or doesn't break anything working

• [x] Build documentation locally
• [x] Open public documentation http://akvo.github.io/akvo-rsr/

Closes #5261

[coveralls]

Coverage: 68.33% (+0.003%) from 68.328% when pulling 44ab1872dac627dc9d35013d7559aade8968a375 on feature/5261-generate-reference-documentation-from-docstrings into 30ba7b2452ff577a8c7456547efd58ecea384165 on master.