Enhance experiment designing documentation

[jaredlockhart]

Summary

The existing "Designing" experiments article is very brief and could use the same enhancement treatment applied to the Rollouts (#778) and Firefox Labs (#776) articles.

Current state

The article is only ~25 lines and links out to a Miro board, an experiment brief template, a Jira ticket, and office hours. It doesn't provide much guidance on its own.

Approach

Expand the article with clearer structure, cross-links to the rollouts and Firefox Labs articles for comparison, and more practical guidance — while keeping it accessible for a non-technical audience. Based on #777 (Firefox Labs) and blocked on #779 (Rollouts).