docs: add v2 upgrade doc

[jescalada]

Closes #1324.

Feel free to add any missing details or correct anything that mind sound weird. This doc was curated 100% manually by going through each release candidate's notes (rc1 through rc4) and adding noteworthy PRs along with some of their details and related issues.

Some of these additions/fixes may have been superseded by another - for example the singleBranch: true performance improvement which got removed in another PR. Let me know if there are any specific ones that need correction to reflect the actual state of v2.0.0!

Note: Once the doc is ready to go, I'll apply a grammar check and improve wording where necessary.

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[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.40%. Comparing base (dd7d162) to head (599b497).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1333   +/-   ##
=======================================
  Coverage   80.40%   80.40%           
=======================================
  Files          65       65           
  Lines        4608     4608           
  Branches      775      775           
=======================================
  Hits         3705     3705           
  Misses        888      888           
  Partials       15       15           

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[kriswest]

Added some corrections and a few comments - many thanks for doing this!