Add update Make targets to project

To round out some of the recently added make targets, add some targets for keeping checkouts of projects up-to-date in a non-destructive way.

The most common reason to need this is having local clones of docs projects fall out of date with the global navigation in gitlab-docs, and wanting a simple way to rectify.

If you know which project is out-of-date, you can call that make target directly.

If you want to update all docs projects, you can do that.

If you want to update all docs projects, and "self", you can do that. You might do that regularly so your default branch for each project is pretty close to upstream all the time.