Document how to use and add to the keywords listing in the tutorial contributions guide

[jonathansick]

The de-facto reference for allowed keywords is in learn-astropy-librarian at https://github.com/astropy/learn-astropy-librarian/blob/main/astropylibrarian/data/keywords.yaml

The contribution guide should tell contributors what keywords are allowed and should also provide a process for how to suggest new keywords.

Note that we'd also like to adopt UAT: see astropy-learn/astropy-tutorials#652

Also see astropy-learn/learn-astropy#20 for a general issue on updating the contribution docs.