namespace documentation in the C++ code

[JaneSullivan-NOAA]

Link to original issue

Title of section
"Use of unique namespaces in the FIMS C++ code"

Suggested section content and location
Subsection in "Contributor Guidelines", either as a subsection of "Naming Conventions" or "Coding Good Practices"

Additional context
Add any other context or screenshots about the feature request here.

Why use unique namespaces? The use of unique namespaces allows developers to quickly identify the source file for a namespace member class when used in a different part of the code and to understand how classes relate to one another throughout the codebase.

When to add a new namespace? Theoretically all new namespaces in the FIMS codebase should fall under the "popdy" namespace (everything inside inst/include/populations_dynamics). However, if new folders are added to inst/include then potentially they warrant a new namespace.

How to document a namespace? As of 10/18/2023 we are still trying to figure out how to document namespaces in doxygen. We want something that looks like TMB's doxygen website, where namespaces are defined in the namespace list and classes are defined as a member of each namespace in the class list.

Collaborators: @k-doering-NOAA @Andrea-Havron-NOAA @ChristineStawitz-NOAA