Update documentation about Collections

[kaskranenburgQ]

At the moment, there is no documentation concerning the MYC.

It might be useful to add this to the documentation, so users can gain understanding around the tool.

[mabijkerk]

There is some, probably outdated, documentation in the repo that does not appear to be included in the docs website:https://github.com/quintel/documentation/tree/master/modules-docs/multi-year-charts.

[kaskranenburgQ]

Assigned to @mabijkerk re-assign at will.