Committer's Guide: four tiers: heading levels

https://docs.freebsd.org/en/articles/committers-guide/#_statement_of_general_intent

Level 3 section title for tiers 1, 2, and 3, and for the unsupported
tier.

Reviewed-by: imp
Approved-by: emaste
Pull-request: https://github.com/freebsd/freebsd-doc/pull/188
Closes: https://github.com/freebsd/freebsd-doc/pull/188