Move documentation deployment to GitHub Actions

We are going to remove Travis CI configuration and we need to move the `DOC` build job from Travis CI to GitHub Actions,
ideally as a separate dedicated workflow.

Shortly, it just needs to replicate steps that we used to run by the scripts that have been removed in commit https://github.com/boostorg/gil/commit/e3e779cc69af4905e4910c06f7e9417c2563e362
- `.ci/upload_docs.sh`
- `.travis.yml`

and that's pretty much it, but on GitHbu Actions.

### Tasklist

- [x] in develop branch
- [ ] in master branch