docs: Set English as language for Sphinx

[matthewfeickert]

Description

Set 'en' - English as the language for the docs. In Sphinx v5.0.0+ setting a language is required to avoid:

WARNING: Invalid configuration value found: 'language = None'

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• Tests are passing
• "WIP" removed from the title of the pull request
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* Set 'en' - English as the language for the docs. In Sphinx v5.0.0+ setting
a language is required to avoid:
> WARNING: Invalid configuration value found: 'language = None'

[matthewfeickert]

As other maintainers are on holiday, I'm going to approve and merge this myself. As always, PRs approved by a single core dev can be reverted as needed by the rest of the dev team.

[codecov]

Codecov Report

Merging #1875 (9ce30e9) into master (fd99262) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1875   +/-   ##
=======================================
  Coverage   98.17%   98.17%           
=======================================
  Files          68       68           
  Lines        4322     4322           
  Branches      726      726           
=======================================
  Hits         4243     4243           
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.39% <ø> (ø)
doctest	60.73% <ø> (ø)
unittests-3.10	96.04% <ø> (ø)
unittests-3.7	96.02% <ø> (ø)

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