Update the "Add New Overlay" instructions for new build system

[bismurphy]

There is an entry at https://github.com/Xeeynamo/sotn-decomp/wiki/Decompilation#add-new-overlay which describes how to add a new overlay to the project. It was made with our old build system and required modifying several files in several places. With our new and improved build system, the process is simpler, and should be documented here.

[hohle]

Removed all of the unnecessary steps, but I thinkmake-config.py could still do a bit more like generating {overlay}.h.