Update redirected links in contributing.md to canonical URLs

[lisancao]

Five external links in contributing.md (and the rendered site/contributing.html) currently return 200 only via redirect to canonical destinations:

Link	New canonical URL
https://help.github.com/articles/fork-a-repo/	https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo
https://help.github.com/articles/using-pull-requests/	https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests
http://legacy.python.org/dev/peps/pep-0008/	https://peps.python.org/pep-0008/
http://www.oracle.com/technetwork/java/codeconvtoc-136057.html	https://www.oracle.com/java/technologies/javase/codeconventions-contents.html
http://docs.scala-lang.org/style/	https://docs.scala-lang.org/style/

GitHub deprecated help.github.com in favor of docs.github.com, Oracle deprecated technetwork, and the two HTTP URLs upgrade to HTTPS. The redirects work today but the canonical URLs are stable forever.

The change touches both contributing.md and site/contributing.html to keep the source and rendered files in sync, matching the convention from PR #679 (Fix a couple of typos in contributing.md).

Generated-by: Claude Opus 4.7 (1M context) (claude-opus-4-7)

[nchammas]

Thanks for updating these links. Could you check them again? It appears a couple of them still redirect to someplace else (perhaps due to the time that has passed since you initially submitted this PR).

Also, you don't need to submit changes to site/ anymore thanks to #699.

[lisancao]

Thanks @nchammas. Refreshed both GitHub links to the current canonical paths:

• fork-a-repo → https://docs.github.com/en/pull-requests/how-tos/work-with-forks/fork-a-repo
• about-pull-requests → https://docs.github.com/en/pull-requests/reference/pull-requests

Verified both return 200 with no redirect. The other three (peps.python.org, oracle, docs.scala-lang.org) still resolve cleanly, so I left them. Also dropped the site/contributing.html change, so this is contributing.md only now, per #699.

[nchammas]

LGTM pending a refresh of the PR description to reflect the current diff. I would also remove the actual before/after URLs from the description since they are already captured in the diff.