Standardize on "docs" subdirectory for documentation

[jphickey]

Is your feature request related to a problem? Please describe.
In nasa/osal#1071 and nasa/PSP#294, the doc subdirectories will be renamed to docs, to be consistent.

Going forward, documentation should be in a docs subdirectory on all submodules/components.

Describe the solution you'd like
Places where CFE build system is using/referring to a doc subdirectory (e.g. refs into OSAL) should be changed to docs

Additional context
Needed for compatibility with/dependency on nasa/osal#1071

Requester Info
Joseph Hickey, Vantage Systems, Inc.