Replace 'master' with 'main' in docs, README files, and comments

Following the recent replacement of the branch main with main, this MR replaces occurrences of references to master with main in documentation, README files, and comments.

Please notice that all references to the master branch of the geomodelatlas repositories have been kept, since those repositories have not been migrated to main yet.

-- Cc: @tsulaia @boudreau

CI/DockerfileFull

 # Setup the most basic image
-ARG BASEIMAGE=gitlab-registry.cern.ch/geomodeldev/geomodel:master-base
+ARG BASEIMAGE=gitlab-registry.cern.ch/geomodeldev/geomodel:main-base
 FROM ${BASEIMAGE}