Unify Developer installation instructions

[mradamcox]

The developer installation in the documentation instructs users to clone the main geoblacklight repository, and install that codebase to begin working. On the other hand, the Readme in the main GeoBlacklight repository recommends using a template from within the main codebase, and not doing a full clone.

I have personally used the latter strategy to begin working on a custom implementation of GeoBlacklight, and I assume the former is how one would go about developing on the main codebase itself. It would be good to have both of these installation strategies in the main documentation repo, with a little context about which approach should be used when.

[karenmajewicz]

Clarify paths - if you want to contribute code vs if you want to create your own app