Build API reference docs from docstrings using MKDocs

[hugovk]

For #139.

Changes proposed in this pull request:

• First part for Add documentation on Read the Docs #139: build the docs from the docstrings (deploy to RTD or gh-pages follow)
• Update most docstrings to follow the same format (still a few to do, can be updated later)
• Also some v2 updates to lint.yml and test.yml

To test locally

pip install -U mkdocs mkdocs-material mkdocstrings
mkdocs build
# or
mkdocs serve

[codecov-commenter]

Codecov Report

Merging #147 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files           9        9           
  Lines         524      524           
=======================================
  Hits          521      521           
  Misses          3        3           

Flag	Coverage Δ
#GHA_Ubuntu	99.42% <100.00%> (ø)
#GHA_Windows	99.42% <100.00%> (ø)
#GHA_macOS	99.42% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/humanize/i18n.py	92.50% <ø> (ø)
src/humanize/number.py	100.00% <ø> (ø)
src/humanize/filesize.py	100.00% <100.00%> (ø)
src/humanize/time.py	100.00% <100.00%> (ø)

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