Update docs CI to match job naming convention

The naming convention for CI jobs is generally stage:name where the stage matches the GitLab stage and the name is a description. However, the documentation jobs broke this convention by being called docs:type where type is continuous, latest, or master. This breaks the convention because docs is not a stage and type does not really describe the build. This fouls up things like reproduce_ci_env.py that expect the first part of the name to be the same as the stage.

This changes the names to build:docs-continuous, build:docs-latest, and build:docs-master. This follows the convention of matching the names with the stage.