Automate release process

[jwodder]

The dandidav release process (documented in doc/releasing.md) involves too many manual steps. As much as possible should be automated.

Goals for the release automation:

• auto is not allowed.
• The automation should take care of creating the tag and GitHub Release.
  • How would signing the tag then be handled?
• The release notes should match the CHANGELOG section for the release, except that multiline paragraphs need to be unwrapped for the former.
• Changelog items/release notes should be hand-crafted, not automatically generated.
• Ideally, releasing should be done via a dedicated release PR that gives the maintainer the chance to review the final changelog items.
  • Tagging & GitHub Release creation happens once this PR is merged.

Possible ways to implement this:

• Use https://github.com/cargo-bins/release-pr to create a release PR with a "release" label, and add a GitHub Actions workflow, triggered on the merge of a "release" PR, that creates the tag and GitHub Release
• Others???