Update contributing guide links

[neverett]

Update links to contributing guides since those have moved.

[eapolinario]

We have this action Register example to sandbox marked as required for all PRs, but that entire CI job is skipped if we're only editing files under the docs directory.

Unfortunately the simple check in github to skip checks altogether is not enough if we want to decrease the "weight" of these docs-only changes. There are ways to skip specific steps (e.g. using https://github.com/dorny/paths-filter), but that complicates the workflow, so to unblock this PR I removed that action as required for the time being while we figure out how to incorporate this in the checks.yml gh workflow.