docs(admin): occ_database completeness pass — dav + db commands

Add all missing dav:* and db:* subcommands with verbose examples and
accurate output samples. Standardize examples to user "layla", clarify
dav:absence:set field semantics (short-message vs message vs
replacement-user-id), document sendEventRemindersMode prerequisite for
dav:send-event-reminders, clarify dav:move-calendar --force behaviour,
and fix dav: heading auto-links via backslash escape.

AI-Assisted-By: claude-sonnet-4-6
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala