workflows: automatically generate a list of versions on deployment

[ES-Alexander]

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.

 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