docs: fix broken contribution workflow link in project-template

[Gaalbu]

Summary

Fixes #126.

The "Understand how to contribute" link in project-template/CONTRIBUTING.md
pointed to https://docs.docker.com/opensource/workflow/make-a-contribution/,
which returns 404.

Investigation

The entire /opensource/ section this page belonged to has been removed from
docs.docker.com — it's absent from the current sitemap and there's no
successor page under that path. A previous attempt at this fix (#127) replaced
the link with https://docs.docker.com/contribute/, but that URL also 404s
(verified with curl), which is likely why it didn't get merged.

Since no equivalent contribution-workflow page currently exists on Docker
Docs, I replaced the link with https://github.com/moby/moby/blob/master/CONTRIBUTING.md
(verified 200 OK) — a live, in-depth contribution guide covering the same
ground (dev setup, PR process, DCO). It's also consistent with the rest of
the paragraph, which already links to docker/docker (auto-redirects to
moby/moby) for the sibling guidelines/triage/review docs.

Testing

• curl -o /dev/null -s -w "%{http_code}" https://docs.docker.com/opensource/workflow/make-a-contribution/ → 404 (confirms original issue)
• curl -o /dev/null -s -w "%{http_code}" https://docs.docker.com/contribute/ → 404 (confirms why docs: fix contribution guide link #127 didn't resolve the issue)
• curl -o /dev/null -s -w "%{http_code}" -L https://github.com/moby/moby/blob/master/CONTRIBUTING.md → 200 (new link works)