doc: fix outdated links in CONTRIBUTING.md

[spnirmal]

Contribution description

This PR updates outdated/deprecated links in CONTRIBUTING.md to ensure new contributors are redirected to the correct resources.

Changes made:

• Updated old GitHub links pointing to deprecated documentation pages.
• Replaced invalid GitHub URLs with current links.
• Improved clarity of instructions by fixing formatting inconsistencies.

This improves documentation quality and reduces friction for new contributors.

Testing procedure

1. Open the modified CONTRIBUTING.md file in the browser or markdown preview.
2. Click on each link that was changed.
3. Verify that all links correctly redirect to the intended, current documentation pages (no 404 / deprecated pages).

No code execution needed — documentation only.

Issues/PRs references

None.

[maribu]

Suggested change

	[git-scm-getting-started]: https://docs.github.com/en/get-started
	[git-scm-getting-started]: https://git-scm.com/book/en/v2/Git-Basics-Getting-a-Git-Repository

This would probably be the fixed URL with the same resource as before. I think it might be better to discuss changing this to Github specific doc in a follow up PR and focus this on just fixing the broken links. That way, we can just merge this without having to discuss changes.

[crasbe]

This got lost in the squash 👀

[spnirmal]

This got lost in the squash 👀

thanks for pointing that out. i have restored the correct link thanks for guidance.

[riot-ci]

Murdock results

✔️ PASSED

1a2b77d doc: fix outdated links in CONTRIBUTING.md

Success	Failures	Total	Runtime
1	0	1	02m:48s

Artifacts

• Documentation preview

[AnnsAnns]

Thank you for the changes :) Please squash

[AnnsAnns]

It looks like you accidentally included the commits that were merged as part of #21823 in your squashed commit

[AnnsAnns]

It looks like you accidentally removed the review change.

For future reference, because git can be confusing. You can use git rebase -i COMMIT_BEFORE_YOUR_PR and then change the pick to fixup on the commit you want to squash into the other commit. https://git-scm.com/docs/git-rebase#_interactive_mode

[spnirmal]

the removal of review change was unintentional - it happened while squashing commits.i have now ensured that all relevant changes are included and only the intended modification remains in this PR.
going forward i will use git rebase -i as suggested.

[crasbe]

Thank you for your contribution! :)

Git and Github can be a steep learning curve indeed.