docs: fix broken contribution guide link

[ukane-philemon]

- What I did
Updated a broken contribution guide link.

- How I did it
Changed https://docs.docker.com/opensource/project/who-written-for/ to https://docs.docker.com/contribute/overview/

- How to verify it
Visit https://docs.docker.com/opensource/project/who-written-for/

- Description for the changelog
Updated a broken contribution guide link.

Closes #4036

[neersighted]

This isn't really the right fix; that contribution guide is the docs contributing guide. See moby/moby#45026 for the equivalent in moby/moby.

These projects used to be in the same repository; I'm not sure what the right solution is here (maybe it's merging moby/moby and linking there), but some sort of restoration of the materials that used to live at that URL will have to happen instead of linking to an unrelated guide.

[ukane-philemon]

Ah, I see. I obviously didn't know what was in the broken link before now but will check out how it was fixed here moby/moby#45026

These projects used to be in the same repository; I'm not sure what the right solution is here (maybe it's merging moby/moby and linking there), but some sort of restoration of the materials that used to live at that URL will have to happen instead of linking to an unrelated guide.

EDIT: I think it is better if we port the contents here to this repo(we might have to tweak a few sentences). Any other thoughts?