Add admin urls to docs - #11963

[smark-1]

This adds documentation about getting URLs for some of the most common admin views #11963. Snippets pages and Images are all documented slightly differently. I am not sure which is the best design.

Rendered: https://wagtail--11965.org.readthedocs.build/en/11965/extending/admin_views.html#adding-links-in-admin-views