Document recommended file naming conventions and expected content

[jphickey]

Is your feature request related to a problem? Please describe.
The CFS apps still vary quite a bit in terms of the content and structure of the source files, even though many do share similar file names in their source trees, the content is not always aligned.

For example, HS and HK apps both have tbldefs.h files (draco-rc4 tag linked here):

https://github.com/nasa/HS/blob/a99b89052d81395df88294d5549bc5cd1a6b703d/fsw/inc/hs_tbldefs.h
https://github.com/nasa/HS/blob/a99b89052d81395df88294d5549bc5cd1a6b703d/fsw/inc/hs_extern_typedefs.h
https://github.com/nasa/HK/blob/b8821d6a65e1d400f0aa99605590a9660de7da4d/fsw/inc/hk_tbldefs.h
https://github.com/nasa/HK/blob/b8821d6a65e1d400f0aa99605590a9660de7da4d/fsw/inc/hk_extern_typedefs.h

But the content of the file between these two apps is quite different; In "HS" the tbldefs.h file contains only some #define statements associated with table definitions, whereas in "HK" this has the complete table definition structures. In contrast, the "HS" app puts the complete table structure definitions in hs_extern_typedefs.h, but this same file has only a #define in HK.

Describe the solution you'd like
Improve consistency not only in file naming but also the content and scope/usage of those files, which starts by documenting the recommended patterns so developers have a guide to follow, as opposed to assumptions.

Additional context
This is extra misleading in CFS because files do have similar-looking names, which suggests similar usage and content, but the latter is not actually the case. Having the appearance/veil of consistency without actual consistency within the file is worse than having no appearance of consistency at all.

Requester Info
Joseph Hickey, Vantage Systems, Inc.