Document how to make facets in solr with real data

[rosiel]

As discussed in Slack, making a Collection facet using field_member_of has some problems. Namely, it's used for collections as well as children, so the facet shows things that aren't collections. Options include creating a custom processor to filter by bundle, or creating a custom solr field that recurses up ancestors.

Let's put this in the docs!

Discussion: https://islandora.slack.com/archives/CM5PPAV28/p1618532028258200
Thanks @elizoller @nigelgbanks @kayakr

[nigelgbanks]

For reference this is partially documented here: https://github.com/Islandora/islandora/tree/8.x-1.x/modules/islandora_advanced_search#configure-collection-search