Add simple meta-documentation to Makefile and README.md

[danudey]

Add documentation for Makefile to Makefile; add documentation for operator reference updates to README.md

Product Version(s): N/A

Issue: N/A

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SME review:

• An SME has approved this change.

DOCS review:

• A member of the docs team has approved this change.

Additional information:

This PR adds two sets of documentation; one, a large comment block at the top of Makefile,
includes a list of the most interesting/relevant Makefile targets for docs building and
for udpating the Operator reference for docs. The second is a small list of the most
interesting targets for updating the Operator reference, and how to use them.

Merge checklist:

• Deploy preview inspected wherever changes were made
• Build completed successfully
• Test have passed

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[ctauchen]

LGTM, thanks. Feel free to merge.