doc: CI: Replace Travis with GitHub Actions
Travis CI is gone, now.  Replace it with GitHub Actions.
# Continous Integration
-libcsp uses Travis-CI App for continous integration:
-<https://github.com/marketplace/travis-ci>.
-
-libcsp on Travis: <https://travis-ci.com/libcsp/libcsp/branches>
+libcsp uses GitHub Actions for continous integration:
+https://github.com/libcsp/libcsp/actions
