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:
- renames "expense" to "expenses" as it is in the app and in the title
- restructures the files
- moves the media files from "media" folder to "expenses" folder
- adds a short intro to expenses
task-2535899
