Changelog: Refactor to single central "pull" GHA workflow

[ewels]

At the moment, there are multiple repositories each with their own GitHub Actions workflow to push a PR to the docs when they mint a new release. This is proving to be a little problematic due to:

• Maintainance burden - multiple repos, unclear responsibilities, differences in available GitHub secrets
• In practice, GitHub release notes are often edited post-release, and the automation misses that

Instead, we should have a single GHA workflow that's in the seqeralabs/docs repo that runs nightly and checks relevant repos for new releases, and creates PRs if any are found. Essentially a "pull" mechanism instead of "push". This should solve the above problems.

Originally suggested by @gavinelder, hat-tip to him 😅

[ewels]

Existing workflows which should be deprecated / removed if we do this:

• Nextflow
• Wave
• Fusion
• MultiQC (with custom script)

So we may want to include the possibility for some automation customisation still, eg. MultiQC. Or we just leave that one alone.