Save Qiskit release notes file for rc releases?

[Eric-Arellano]

We intentionally do not save the release notes file for dev releases because we don't want unreleased notes to show up. But rc1 is an official release, so it would be helpful to have that release note included.

Note that implementing this is somewhat complex because normally only the stable release changes release notes. We'd need to make sure that the stable release does not accidentally delete the rc1 release notes file.

This should only happen when the version is .rc, not -dev.

Update May 9, 2024: we think this is a good idea, but the left ToC should make clear it's rc1 / not stable. And page title.