Document deprecation process

Is your feature request related to a problem? Please describe.
Process not documented

Describe the solution you'd like
Document process and reference.

• Issue requesting deprecation gets reviewed by CCB, accepted/assigned or rejected
  • evaluate requirements/documentation/operational impacts
• Pull request(s) wraps deprecated elements in *_OMIT_DEPRECATED ifndef and removes any internal dependencies
  • Default sample configuration does not remove this code
  • make OMIT_DEPRECATED=true prep will compile without it to check code for compliance
  • ideally any deprecated elements address dependency fix (use new element, no longer supported, etc)
  • elements can be deprecated in any build/release
  • CI tests with and without deprecated elements removed to ensure no internal dependencies
• Issue requesting removal of deprecated code
• Pull request removes deprecated elements
  • typically done end of cycle prior to a major release

Describe alternatives you've considered
Wing it.

Additional context
None

Requester Info
Jacob Hageman - NASA/GSFC

No documentation file is named. Start with the deprecation workflow in this issue, then locate the project documentation and inspect the *_OMIT_DEPRECATED guards, make OMIT_DEPRECATED=true path, and CI coverage described here. Done means the review, deprecation, removal, release timing, and validation steps are documented with their operational impacts.