Add section to Developer's Guide to provide guidance on code structure

[glemieux]

Is your feature request related to a problem? Please describe.
While we have a style guide for providing guidance on formatting details, we don't provide guidance on how to structure new code additions (e.g. explaining why certain modules live where they do and where to place new modules).

Describe the solution you'd like
Add a new section under "Overview" containing multiple headings to address the following:

• fates directory structure and module placement
• where to place named constants (EDParams vs. FatesConstants vs. ?)
• when to develop a new type or add to an existing type
• passing as arguments versus "using"
• etc.

Describe alternatives you've considered
I considered that we could add this to the style guide, but I think its a larger issue than style. If the concept being address is significantly large enough, we could give it a subsection of it's own.

Additional context
This was inspired by @adrifoster work on NGEET/fates#1024 which refactors the code structure.