Clarify CONTRIBUTING.md documentation generation steps (#3443)
Clarify CONTRIBUTING.md documentation generation steps (#3443)

* Clarify document generation steps

Signed-off-by: Dwayne Robinson <68b3e4a38f31943d1252e7fe5353e8bd22634884@microsoft.com>

* Add note about stat_coverage.py, which is undocumented but also essential for the checkin to succeed

Signed-off-by: Dwayne Robinson <68b3e4a38f31943d1252e7fe5353e8bd22634884@microsoft.com>

Co-authored-by: Chun-Wei Chen <3d21e1cad9aa0e4aa10838f859be24690cfc357c@gmail.com>
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