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 "Sign" app:
- it removes a useless "overview" category
- it adds a brief explanation at the "app" level, including links to
relevant docs
task-2535899
