Update docs

[hugovk]

Changes proposed in this pull request:

• Update README following PR Update minimum_unit handling of naturaldelta and naturaltime #142
• Link RTD's API reference and README's usage
• Add pydocstyle to lint and fix the findings
• Add some missing returns/raises (found via https://github.com/terrencepreilly/darglint, not added to lint as there's more to address in the future)

Also:

• Switch from flaky codecov-bash to codecov-action

[codecov-commenter]

Codecov Report

Merging #153 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #153   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          556       556           
=========================================
  Hits           556       556           

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

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

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