Release v0.6.3

[sethrj]

Pre-merge checklist

• [x] Ensure all CI jobs on the branch to be released (develop or backports/vX.Y) pass.
• [ ] ~~Run performance regression tests on Perlmutter, Frontier, and an additional machine with debug assertions enabled (e.g., Wildstyle).~~
• [x] Update documentation with release notes from all pull requests newly included in the release.
• [x] Ensure PDF documentation builds without error.

Post-merge checklist

• [ ] If releasing a backported version branch, cherry-pick this documentation commit into the backport branch.
• [ ] Use the release scripts to:
  • Add a text description of the release based on the change set
  • Draft a new GitHub release
  • (then, using the GitHub web interface) Publish the release
  • Draft a zenodo release
  • (then, using the Zenodo web interface) Publish the zenodo citation

Post-release checklist

• [ ] Use the release script to update the archive.
• [ ] Pull locally (make sure to use the --tags option) and build PDF user documentation for the release, and upload it. Ensure breathe is activated (so the API is listed) and that the version is embedded correctly.
• [ ] Update the Spack recipe for Celeritas with the new version and the sha256 value returned by the release script. Submit a pull request to the Spack project.
• [ ] Mark the GitHub release milestone as completed.