Document image server management

https://redmine.teklia.com/issues/10979

Under a new Images section, new pages should be added:

• Overview: the fact that we use IIIF, and what images and image servers are
• Creating an image server: the server detection algorithm in CreateIIIFURL, the errors to expect from IIIF imports, and how to create an image server manually through the admin if that fails
• Managing image servers: how to move images around using the split/merge buttons in the Django admin to handle servers that moved around or a "matched multiple image servers" issue

This blog post from 2021 already contains most of the necessary knowledge, though that should be checked from the source code as there have been some updates ever since.

#13 can then be a follow-up to this