Add AGL docs page to make a custom recipe

Add AGL docs page to make a custom recipe

This docs page shows how to add a custom recipe from a git-repo.
It shows how to do this step by step.
It was used to make the cannelloni recipe.

Bug-AGL: [SPEC-3768]

Signed-off-by: Thomas Walzer <thomas.walzer@reutlingen-university.de>
Change-Id: I306786bfc42aa64eb256259f61d0e402e23c37a2
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25907
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>