move add-on dev section one level up

[intoeetive]

The proposed change is moving "Add-on development" from subsection in "advanced usage" to its own parent menu section

There are too many depth levels to navigate in it, so I think bringing it one level up is making it better experience for everyone. Specifically, docs on service and models are much more accessible now.

Also grouped "v3-4-6 migration" pages under "Updating old add-ons"

[intoeetive]

@TomJaeger this is desperately needing your approval