ci(release): fix release tagging, misc cleanup

[wpbonelli]

• ame-yu/action-delete-latest-release is archived, use gh CLI instead
  • this should fix the release tagging, which is broken — current points to a commit from long ago
  • at some point, maybe ought to consider tagging examples releases to match MF6 release versions? users have reported some difficulty discovering which mf6/examples/flopy versions are compatible — after discussion with @langevin-usgs I have switched all three RTD site default versions to stable to try to head off some confusion, but this site needs to be triggered from actions to properly render as we build site assets here and upload them to RTD, see item below
• add workflow dispatch triggers for manual testing and to allow rebuilding the RTD site manually
• make release asset names consistent, this will require a corresponding update in mf6
  • mf6examples.pdf
  • mf6examples.zip
• factor out some environment variables in ex-workflow.yml
• miscellaneous cleanup, tidying