Investigate 'gh-pages' package instead of Travis deploy

[jaeddy]

I saw the gh-pages libraries mentioned in a Redoc-related issue for use with publishing rendered docs to GitHub Pages. Using the package might be safer / easier to control than the blanket pages deploy with TravisCI (and would also be more build system agnostic).

[tschaffter]

@jaeddy I propose instead to publish documentation to gh-pages using GH Actions as part of the CI/CD workflow. We could use the solution that I used for the NLP Sandbox Schemas repository.