Add particl checks and references to unit-e checks

[scravy]

This adds references to additional checks in particl (complete) and links a few points in unit-e (not complete, some things are still to be worked on).

[scravy]

What is the difference between docs and documentation? Is docs for documents? In unit-d-docs?

[cornelius]

What is the difference between docs and documentation? Is docs for documents? In unit-d-docs?

docs is a leftover from consolidating the labels across repositories. We use documentation for labeling everything related to documentation.

I'll clean it up here as well and merge the labels.

[scravy]

@Ruteri @Gnappuraz I addressed the comments, please have another look.