docs/CONTRIBUTING.md: Fix links in CONTRIBUTING

Get them to point to the rendered version, instead of
letting Github thinks they are absolute links in the
repository. The rendered version is much more user-friendly.

docs/CONTRIBUTING.md

@@ -33,13 +33,13 @@ For older versions that are still supported by your distribution please use resp
 
 ## Security vulnerability reports
 
-See [reporting of security vulnerabilities](/SECURITY).
+See [reporting of security vulnerabilities](https://systemd.io/SECURITY).
 
 ## Posting Pull Requests
 
 * Make sure to post PRs only relative to a recent tip of the `main` branch.
-* Follow our [Coding Style](/CODING_STYLE) when contributing code. This is a requirement for all code we merge.
-* Please make sure to test your change before submitting the PR. See the [Hacking guide](/HACKING) for details on how to do this.
+* Follow our [Coding Style](https://systemd.io/CODING_STYLE) when contributing code. This is a requirement for all code we merge.
+* Please make sure to test your change before submitting the PR. See the [Hacking guide](https://systemd.io/HACKING) for details on how to do this.
 * Make sure to run the test suite locally, before posting your PR. We use a CI system, meaning we don't even look at your PR if the build and tests don't pass.
 * If you need to update the code in an existing PR, force-push into the same branch, overriding old commits with new versions.
 * After you have pushed a new version, add a comment explaining the latest changes.