[DOCS] Overhaul docs versioning

[ChrisChinchilla]

This PR moves docs versions to Hugo themes (currently in the themes directory, which is odd, but that's the way it works…). Still do to:

• [x] Docs on releasing a version
• [ ] Add new versions
• [ ] Add Netlify redirect rule
• [ ] Fix tests
• [x] Fix menu
• [x] Fix git edit link

[codecov[bot]]

Codecov Report

Merging #3945 (b013082) into master (b013082) will not change coverage. The diff coverage is n/a.

:exclamation: Current head b013082 differs from pull request most recent head 199cfa3. Consider uploading reports for the commit 199cfa3 to get more accurate results

@@          Coverage Diff           @@
##           master   #3945   +/-   ##
======================================
  Coverage    56.7%   56.7%           
======================================
  Files         555     555           
  Lines       63427   63427           
======================================
  Hits        36022   36022           
  Misses      24218   24218           
  Partials     3187    3187           

Flag	Coverage Δ
aggregator	62.6% <0.0%> (ø)
cluster	∅ <0.0%> (∅)
collector	58.4% <0.0%> (ø)
dbnode	60.5% <0.0%> (ø)
m3em	46.4% <0.0%> (ø)
metrics	19.7% <0.0%> (ø)
msg	74.1% <0.0%> (ø)

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