Update search links in Rate Limits documentation

[rellem]

Rationale:

• crt.sh is a great resource, but it seems to have server issues quite often.
• Censys is very powerful, but also quite complicated for this use case. They also have limitations for anonymous users.
• CertObserver CT search is fast, free, and doesn't require sign-up. It also groups related certificates, making the results easier to scan.

Disclaimer: I'm the founder of CertObserver.