docs: add release step to update pinned charm tests (canonical#1213)

We will switch to using pinned versions of the charm (latest `main`) for
our charm tests in the GitHub Actions. This PR adds a release step to
update the pinned versions, and what to do if they fail.

---------

Co-authored-by: Tony Meyer <tony.meyer@canonical.com>
Co-authored-by: Dima Tisnek <dima.tisnek@canonical.com>

HACKING.md

@@ -293,13 +293,16 @@ To make a release of the ops library, do the following:
    link to the full changelog.
 8. Change [version.py][ops/version.py]'s `version` to the
    [appropriate string](https://semver.org/).
-9. Add, commit, and push, and open a PR to get the changelog and version bump
+9. Check if there's a `chore: update charm pins` auto-generated PR in the queue. If it looks
+   good, merge it and check that tests still pass. If needed, you can re-trigger the
+   `Update Charm Pins` workflow manually to ensure latest charms and ops get tested.
+10. Add, commit, and push, and open a PR to get the changelog and version bump
    into main (and get it merged).
-10. Back in the GitHub releases page, tweak the release notes - for example,
+11. Back in the GitHub releases page, tweak the release notes - for example,
    you might want to have a short paragraph at the intro on particularly
    noteworthy changes.
-11. Have someone else in the Charm-Tech team proofread the release notes.
-12. When you are ready, click "Publish". (If you are not ready, click "Save as Draft".)
+12. Have someone else in the Charm-Tech team proofread the release notes.
+13. When you are ready, click "Publish". (If you are not ready, click "Save as Draft".)
 
 This will trigger an automatic build for the Python package and publish it to
 [PyPI](https://pypi.org/project/ops/)) (authorisation is handled via a