Move from Travis to github actions

[gwarf]

Move from Travis to github actions: we are experiencing long build delay using Travis, and according to changes in Travis management/strategy it may be safer to start looking at alternatives like GitHub actions.
https://docs.github.com/en/free-pro-team@latest/actions/quickstart

https://developer.okta.com/blog/2020/05/18/travis-ci-to-github-actions

https://github.com/marketplace/actions/hugo-setup

To be done:

• Mardkown linting
• Building site with hugo
• deploy on PR merge

[gwarf]

Done.