meta: Proposals should link back to the main PRs that make them implemented?

[EricCousineau-TRI]

Citing stuff in gazebosim/sdformat#275 was kinda painful.
It would be nice if this were more explicitly discoverable, rather than having to sleuth.

Ideal outcome:
The current proposal_format.md gets a section below "Status", that has links to the PRs / commits that make the proposal fully "implemented".

[EricCousineau-TRI]

For example, PEP's have links to their resolutions:
https://www.python.org/dev/peps/pep-0562/
https://mail.python.org/pipermail/python-dev/2017-December/151033.html

However, it's hard to trace the PR forward from the PEP / resolution email to the actual implementation (not reference implementation).