Automate Release Process

[ax3l]

It would be good if we document and can automate the release process as muhc as possible.

Currently, the release workflow is:

1. update the version in the source tree -- maybe a release script for this? Maybe a GH action, too.
2. tag / GitHub release
3. upload to PICMI: this can be a GitHub action
4. approve the conda-forge PR for the new release (after a few hours/day) -- this is anyway done as part of the WarpX release process
5. Spack PR -- this is anyway done as part of the WarpX release process, but could be more automated