Migrate documentation deployment from Travis CI to GitHub Actions

[seisman]

Description of proposed changes

This is the last step of the CI migration from Travis to GitHub Actions.

The codes are copied from https://github.com/fatiando/boule/blob/master/.github/workflows/docs.yml.

For testing purpose, the deployment steps are enabled for PRs temporarily, and the documentation is deployed to the gh-pages-testing branch.

It seems working very well (see https://github.com/GenericMappingTools/pygmt/tree/gh-pages-testing).

Fixes #566.

TODO

• Revert the changes in commit 4ed4de6
• Remove .travis.yml
• Remove the Travis badge in README
• Update CONTRIBUTING.md
• Update MAINTENANCE.md

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically

[seisman]

Also trigger builds for tags/releases.

[seisman]

Ping @GenericMappingTools/python for review.

[weiji14]

Sorry for the delay in review. A quick scan and this seems fine to me, though I would prefer if @leouieda (who has more experience on Github Actions docs deploy) take another look. Otherwise, I think you can make the necessary pre-merge changes mentioned at https://github.com/GenericMappingTools/pygmt/pull/713/files#r535384202 and other places.