Create github search tips for the CVE Program

[taladrane]

This pull request adds a comprehensive guide, github-search-tips.md, to help users effectively search for CVE Program data across the CVEProject GitHub organization. The document provides practical search techniques, repository-specific tips, and examples for finding CVE Records, rules, policies, API information, and community discussions.

[jgamblin]

Thanks for putting this guide together. It is super useful and easy to follow.

One quick question on the biggest reader-clarity point: would you be open to splitting Code Search examples from Issue/Discussion search examples (or adding a short note where they mix)?

I am asking only from a copy/paste usability angle for readers, since mixed examples can lead to confusing results depending on which search mode they run.