This commit is part of a larger work to improve the documentation's
structure. The toctree structure used to add levels to create categories
and make the structure visually more understandable.
With the new version of the documentation, plenty of these categories
are now redundant.
This commit improves the structure of the "Subscriptions" app:
- it removes a useless "configuration" category and moves its two docs
to the top-level TOC page
- it adds a brief explanation at the "app" level, including links to
relevant docs
task-2535899
83e4b6c816