doc: add FTS5 requirement for sqlite backend. Document the requirement for FTS5 if you are using the sqlite backend. Tonu Mikk reported issues with using a SQLite that was missing FTS5 support (RedHat 7 sqlite with python 3.6). Even though FTS5 has been in the sqlite amalgamation since 2015-10-14 (v3.9.0) RedHat chose to not build their package with that. This leads to a traceback when initializing the database.

try:
  # used for python2 and python 3 < 3.13
  import cgi as cgi
  from cgi import FieldStorage, MiniFieldStorage
except ImportError:
  # use for python3 >= 3.13
  from roundup.anypy.vendored import cgi
  from roundup.anypy.vendored.cgi import FieldStorage, MiniFieldStorage