workflows: automatically generate a list of versions on deployment

Useful for the theme to be able to know which versions can be switched between.

Could maybe add to standard deployment pipeline, just run every time a new change is merged (a bit excessive, but not _that_ wasteful).

e.g.

```yml
steps:
- uses: octokit/request-action@v2.x
id: list_branches
with:
route: GET /repos/{owner}/{repo}/branches
owner: bluerobotics
repo: BlueOS-docs
env:
GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
- run: echo "${{ steps.get_latest_release.outputs.data }}" > branches.json
- run: aws s3 cp branches.json s3://br-www-docs/docs/available-versions.json
```