Add reduced complexity section to the developer's guide

[glemieux]

Is your feature request related to a problem? Please describe.
The reduced complexity modes add complexity to the way fates handles patch areas. I find that I'm frequently having to revisit emails and code comments to correctly remember why we do things in a particular way. It'd be good to collect all that information in one space for developers to reference to gain a better understanding of how to maintain and build new code within this context.

Describe the solution you'd like
Add a section discussing the distinctions between the reduced complexity modes and full fates from a software perspective.

Describe alternatives you've considered
Code comments are good, but are distributed throughout the code by their nature. I'd like to consolidate it into one page in the developer's guide. The user's guide focuses on the scientific rationale for the mode and its expected inputs/outputs, not necessarily how it works under-the-hood.

[glemieux]

@rosiealice do you think you could take a crack at this?

[glemieux]

This could be cribbed somewhat from NGEET/fates-docs#34.