Taiga 728: Clean up FAQ chapter

Taiga 728: Clean up FAQ chapter

In preparation of moving more FAQ from the website
into the manual, this cleans up the FAQ page a bit updating
and removing outdated information.

Entries removed:
- Subtitle display on bibliographic records
  This feature only works with non-XSLT views that are deprecated.
  The Keyword-to-MARC mapping is also in discussion to be removed.
  Description was incomplete.
- Duplicate overdue notices
  This was a bug fixed a long time ago.
- Results limited
  There is no such limit on reports results anymore.
- FRBRizing content
  XISBN feature described here is dead and has since be removed.
- Errors in Zebra cron
  Not written for tools currently used (packages, indexer).