docs: fix remaining broken links in contributing docs

Replace two dead docs.docker.com links with correct in-repo paths:

- CONTRIBUTING.md: image link pointed to removed
  docs.docker.com/opensource/project/who-written-for/ page; now
  links to docs/contributing/who-written-for.md
- .github/PULL_REQUEST_TEMPLATE.md: contributing guide link pointed
  to removed docs.docker.com/opensource/code/ page; now links to
  docs/contributing/ on GitHub

Follow-up to #52228 which fixed most broken links but missed these two.

Fixes #45024

Signed-off-by: Kit Dallege <xaum.io@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,7 +5,7 @@ https://github.com/moby/moby/blob/master/CONTRIBUTING.md
 ** Make sure all your commits include a signature generated with `git commit -s` **
 
 For additional information on our contributing process, read our contributing
-guide https://docs.docker.com/opensource/code/
+guide https://github.com/moby/moby/blob/master/docs/contributing/
 
 If this is a bug fix, make sure your description includes "fixes #xxxx", or
 "closes #xxxx"

CONTRIBUTING.md

@@ -4,7 +4,7 @@ Want to hack on the Moby Project? Awesome! We have a contributor's guide that ex
 [setting up a development environment and the contribution
 process](docs/contributing/). 
 
-[![Contributors guide](docs/static_files/contributors.png)](https://docs.docker.com/opensource/project/who-written-for/)
+[![Contributors guide](docs/static_files/contributors.png)](docs/contributing/who-written-for.md)
 
 This page contains information about reporting issues as well as some tips and
 guidelines useful to experienced open source contributors. Finally, make sure