Switch to Github action

[aniewielska]

This PR changes the CI engine for automatic documentation build from Travis to Github Actions,
Inspired by similar initiatives for TRS and especially DRS.

Points to consider:

• the build is only going to happen on push to any of the branches. The original build process was additionally fired for PRs, but did not commit built docs to Github pages.
• It seems that building and publishing docs for PRs was not taken into consideration by gh-openapi-docs. Introducing it for PRs from branches would be straightforward, for instance by publishing preview of docs under a PR number, but brings not much value (branches are built and published anyway).
• Introducing building and publishing docs for PRs from forks is a potential security issue and requires bypassing GH actions built-in security measures (more here).
• If the build without publishing was supposed to work as a validator for PRs, it would be probably a better idea to introduce a dedicated validation GH Action (on my TODO list).
• contributing doc needs an update, as it still mentions Travis there.
• there is a Travis badge in read.me.
• in order to have all the existing PRs where the build failed built in the new system, this PR should be merged first (which branch?) and all the other branches rebased on top of it.