Add Documentation module

[raar1]

This follows on from the work in PR #31 by:

• Rebasing onto main (the branch was very old and incompatible with the newer versions of NEBULA)
• Adding metadata for the module, including a thumbnail from unsplash (unsplash licensed)
• Adding metadata for the slides and exercises, fixing ordering etc
• Adding learning objectives in info.md
• Moving introduction content to a "Further reading" tab, since it was mostly headings and links. We can add an intro back again later if desired

Fixes #7

[raar1]

OK, I have now added quite a few extra slides from https://esciencecenter-digital-skills.github.io/digital-skills-slides/ regarding documentation because I think they are quite nice and fit here.

Regarding Sven's remark about the need for an introduction, we can still provide one but IMO the slide deck is now essentially a suitable intro to the module and I've placed it first.

I removed the RS-DAT and Xenon examples that Jaro took exception to (I agreed).

This module can certainly benefit from some polishing, additions etc but I think that it is time to merge this as the first version of the Documentation module so that instructors can start preparing. For any further improvements I propose that we open specific issues to handle these. What do you think @JaroCamphuijsen ?