Proposal: Guidance on link within samples

### Description

At the moment, we have no guidance on providing links to other sources within the samples. While they're extremely valuable to have, it can hinder readability of the entire snippet as there would be horizontal scrollbars and other rendering issues such as rendering a long link on the Sample Browser page (example: https://cloud.google.com/secret-manager/docs/samples/secretmanager-access-secret-version).

### Impact

Having proper guidance will ensure that the samples continue to appear rendering correctly with valuable resources being added.

### Challenge

As this involves the team beyond Samples committee, we might have to reach out to the team governing Sample Browser. Luckily, I'm in that team, so I can bring forward any suggestions from this work to the appropriate personnel and/or make the improvements myself on Sample Browser.