Add github action to automate updates to releases pages

The "Releases" pages contain a mirror of the information typically found in the GitHub release pages for each application. This information is reproduced within the docs pages to ensure people can find it without having to go off-site to GitHub when looking for this dynamic information.

The script `scripts/generate-release-notes.sh` wraps `scripts/release-to-hugo.py` to generate the pages:

```
$ scripts/generate-release-notes.sh
Processing syft with weight 10...
Fetching releases from anchore/syft on GitHub...
Found 213 releases
Skipping existing release: v1.32.0 (already exists)
Skipping existing release: v1.31.0 (already exists)
Skipping existing release: v1.30.0 (already exists)
Skipping existing release: v1.29.1 (already exists)

Skipping existing release: v0.1.0-beta.5 (already exists)
Skipping existing release: v0.1.0-beta.4 (already exists)
Skipping existing release: v0.1.0-beta.3 (already exists)
Skipping existing release: v0.1.0-beta.2 (already exists)
Skipping existing release: v0.1.0-beta.1 (already exists)
Done!
✅ Successfully generated release notes for syft
-----------------------------------
Processing grype with weight 20...
Fetching releases from anchore/grype on GitHub...
Found 174 releases
Skipping existing release: v0.99.1 (already exists)
Skipping existing release: v0.99.0 (already exists)
Skipping existing release: v0.98.0 (already exists)
Skipping existing release: v0.97.2 (already exists)
```

etc.

It only generates if the existing markdown file isn't already there. If the `content/docs/releases/*` is removed, then the script will re-generate all of the release notes files. This could be useful if the code/format of the release notes needs to change.

We should replace this with a GitHub action that could be run regularly and generate a pull request when files are updated. It could also be triggered somehow by the releases of our OSS tools.

Start by reading scripts/generate-release-notes.sh and scripts/release-to-hugo.py to understand how release pages are generated and skipped when they already exist. The work is done when a GitHub Action runs the generation on a regular or release-triggered basis and opens a pull request when release-page files are updated.

Written by the indexing model from the issue text.