[0.12.0] backport upgrade guide to release docs

[redshiftzero]

Status

Ready for review

Description of Changes

Changes proposed in this pull request:

• Backports commits in Add 0.11.1 to 0.12.0 upgrade docs #4182

Testing

Ensure same commits as #4182

Deployment

Docs only

Checklist

If you made changes to documentation:

• Doc linting (make docs-lint) passed locally

[emkll]

✔️ Verified commits are the same as #4182

[codecov-io]

Codecov Report

Merging #4199 into release/0.12.0 will not change coverage.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           release/0.12.0    #4199   +/-   ##
===============================================
  Coverage           84.79%   84.79%           
===============================================
  Files                  43       43           
  Lines                2782     2782           
  Branches              303      303           
===============================================
  Hits                 2359     2359           
  Misses                355      355           
  Partials               68       68

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