Prepare 0.6 release

[caleb-johnson]

Remaining action items:

• move release notes to 0.6 directory
• build docs locally; proofread the release notes
• merge this PR
• tag the release
  1. git checkout main
  2. git pull
  3. git show (verify it's the commit we want to tag)
  4. git tag -a 0.6.0 -m "Circuit Knitting Toolbox 0.6.0"
  5. git show (double/triple check the tag is on the correct branch)
  6. git push origin 0.6.0
• Wait for CI to finish. The release should then be available on GitHub and pypi.
• create the stable/0.6 branch
  1. start with same commit as above checked out
  2. git checkout -b stable/0.6
  3. git show (check before pushing)
  4. git push --set-upstream origin stable/0.6
• ensure .mergify.yml, pyproject.toml, and docs workflow versions are all updated
• get docs to deploy from the new stable branch (should happen automatically at this point after CI runs both on the new stable branch and then on the gh-pages branch)

Previous release PR: #438

[coveralls]

Pull Request Test Coverage Report for Build 7876195488

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.541%

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Totals
Change from base Build 7875509586:	0.0%
Covered Lines:	2346
Relevant Lines:	2508

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💛 - Coveralls