Add a section to the Developer's Guide to include an HLM-FATES API design document and standards/guidelines

[glemieux]

Is your feature request related to a problem? Please describe.
As we work to refactor the API over time, we should incorporate some documentation to define the design principles and guidelines.

Describe the solution you'd like
A single section with a design document should suffice.

Describe alternatives you've considered
We could explain the design outline in interface modules in the various HLMs, but I think it'd be better to have a single location describing the overall goal and any HLM-specific requirements to provide ease of maintenance.

Additional context
None