Fix triangulation with CDT and update migration guide

[iche033]

🦟 Bug fix

Summary

Changes:

• Fixes triangle direction. The triangles returned from CDT are counter-clockwise, so convert them to clockwise in submesh.
• Fixes style, lint issues
• Updated migration guide.

I have not moved CDT to its own vendor subdir yet. I just added a README.md with url and commit instead, similar to VHACD.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[scpeters]

I've added a test in a PR targeting this branch: #624