Update changelog actions to include pre-release changelog entries in the stable release changelog.

[moe-ad]

Currently, if a project goes through a pre-release cycle, the changelog action treats the pre-releases like normal releases. This means, for example, changelog entries already in pre-releases 1.1.0a0, 1.1.0a1, ..., 1.1.0rc0, ... will not show up in the entries of stable release 1.1.0. A user consulting the release notes will expect all changes in 1.1.0 to be included in its own changelog, even if they already show up in pre-releases' changelogs.

A concrete example can be found in the release notes of ansys/actions v10.1.0a0 and v10.1.0, where v10.1.0 changelog did not include that of v10.1.0a0.

At a very high level, I guess we need to add extra steps to doc-deploy-changelog that does the ffl when making a stable release:

• parses doc/source/changelog.rst to determine if pre-releases corresponding to the stable release being made exist.
• include their (pre-releases) changelog entries in that of the stable release.

See also ansys/pygranta#113 (the section on release notes)