[User Guide] Reorganize to prepare for other languages

[imnasnainaec]

Setup for #868

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This change is 

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.03 🎉

Comparison is base (a517677) 61.17% compared to head (28f2bf8) 61.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2354      +/-   ##
==========================================
+ Coverage   61.17%   61.21%   +0.03%     
==========================================
  Files         238      238              
  Lines        8389     8389              
  Branches      520      520              
==========================================
+ Hits         5132     5135       +3     
+ Misses       2876     2873       -3     
  Partials      381      381              

Flag	Coverage Δ
backend	73.83% <ø> (+0.06%)	⬆️
frontend	47.26% <ø> (ø)

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see 1 file with indirect coverage changes

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[jmgrady]

Reviewed 43 of 43 files at r1, 35 of 35 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)

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docs/user_guide/config/en/mkdocs.yml line 4 at r1 (raw file):

theme:
  name: material
  custom_dir: "../../overrides/"

Per our discussion on Slack, we may want a name that communicates better to maintainers.

Code quote:

  custom_dir: "../../overrides/"