docs: document Fatras barcode identifiers

[hadronic-arvind]

Closes #2796

[andiwand]

thanks @hadronic-arvind ! this reads well and represents what is in the code right now

I am not sure about the location and what to do with the "old" file. lets wait for @paulgessinger to have a look

[andiwand]

@paulgessinger this file for example we do not show at all right now? is the correct replacement docs/groups/fatras.md, added in this PR? in this case we should migrate more content from old to new and not touch the old one

[hadronic-arvind]

@andiwand do you mean the location of the documentation for the Barcode within the docs/groups/fatras.md‎ or the location of the document itself? Also, if needed, I can help move content from the old fatras.md to the new (after checking if relevant content is up-to-date).

Also, looks like docs/groups/fatras.md feeds the current Doxygen docs, while docs/old/fatras/fatras.md still matters because readthedocs.yml points through the old docs stack. Since the original issue linked ReadTheDocs, I figured updating only the new docs would have been incomplete.

[paulgessinger]

This file indeed is only referenced in the old documentation. It's an open question if we want to push this update there. To be honest, I would probably think: no.