Move docs CI/CD from Travis to Github Actions

[rafaellehmkuhl]

• Changes in the path of the images were needed to fix mkdocs building (refer to this comment for instructions on how to succefully do relative importing on mkdocs, locally and on gh-pages)
• Using html-proofer, like before
• Deployment only runs when the documentation is merged in docs, so nothing is deployed during PRs

[patrickelectric]

Fix #953