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:
- removes the main "settings" section
- move its content to general/odoo_basics
- moves "general/mobile" content to "general"
- moves the content of the doc "developer_mode/activate" to "general"
- it replaces the references to that doc with `:ref:` directives
rather than `:doc:` directives
- it is worth noting that it also fixes plenty of links that
previously referred to "developer_mode" (the category) rather than
"developer_mode/activate" (the document)
task-2535899
