migrate to GitHub actions

[jb-adams]

Solution for #378

Migrates CI processes (i.e. documentation build) to Github Actions from Travis CI.

Documentation: link. There are no spec changes, this is only to show that the docs get built in the same way following the migration

[aniewielska]

@jb-adams - a very nice CI. Where will I find the Dockerfile for gh-openapi-docs? Would it be possible to re-use the dockerized approach for gh-openapi-docs for other APIs?
Also I think you mentioned it, but would this CI actually work for both PRs from branch and PRs from fork? I assume, it will not for the latter due to Github read-only permissions for pull_request, right?