Connect together Research Computing websites

[Sparrow0hawk]

This website refers to arcdocs via the Documentation nav but our arctraining websites aren't linked on either arcleeds or arcdocs.

A good small piece of work would look to:

• - Include the arctraining.github.io site into the training/ section, maybe linked on each course page?
• - Include a training chapter on arcdocs linking to arctraining.github.io (need to create an issue over on arcdocs!)