Document process to customize GeoNode L&F beyond basics

[simboss]

Using the work done for Krihs as a reference we shall prepare proper internal documentation to guide other front-end developers on how to do the same.

[allyoucanmap]

@simboss here the document https://docs.google.com/document/d/1QwboFFXnK8xsED1WWAQ1jUGQ2HxjdxL-lOlY_Qu9L58/edit?usp=sharing

[simboss]

thx @allyoucanmap