Deploy docs to Read the Docs

[hugovk]

For #139.

Changes proposed in this pull request:

• Follow on from Build API reference docs from docstrings using MKDocs #147
• Most of the second part for Add documentation on Read the Docs #139: deploy to Read the Docs
• https://humanize.readthedocs.io is already registered for this project, and I'll ask for access, but in the meantime I've made https://python-humanize.readthedocs.io to get the config all in order

[codecov-commenter]

Codecov Report

Merging #148 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #148   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files           9        9           
  Lines         524      524           
=======================================
  Hits          521      521           
  Misses          3        3           

Flag	Coverage Δ
#GHA_Ubuntu	99.42% <ø> (ø)
#GHA_Windows	99.42% <ø> (ø)
#GHA_macOS	99.42% <ø> (ø)

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