Bug 35463: More preservation manual links

Bug 35463: More preservation manual links

This patch adds some missing links from the preservation module to the manual.

To test:
1. Go to Preservation > Trains
2. Click on a train name
   --> You are brought to the Show train section of the Preservation chapter of the manual
3. Click Edit next to an item
   --> You are brought to the Add items section of the Preservation chapter of the manual (there is no section specific to editing items, but the form to edit is the same as the form to add)
4. Go back to the trains list, and click the Edit button next to one of the trains
   --> You are brought to the Trains section of the Preservation chapter of the manual (there is no section specific to editing trains yet)

Signed-off-by: Philip Orr <philip.orr@lmscloud.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>