[Verify API] Document common scenarios where Verify API is expected to error

Our existing dapp-facing Verify docs provide information on how to verify their prod domain, but do not outline common scenarios where the Verify iframe will fail when loaded, even if they already verified their prod domain.
An example of this is: running the dapp on a preview environment (e.g. *.vercel.app) which will fail the frame-ancestors check for their verified *.myproductiondomain.com
- Deeper investigation and context around this problem can be found here: https://walletconnect.slack.com/archives/C03TFK9BSGJ/p1707297834577739?thread_ts=1706179684.032179&cid=C03TFK9BSGJ
Another example: what does it mean when I get a 404 back from verify.walletconnect.com/attestation ?
- A: 404 when wallet tries to fetch attestations means the dapp doesn't have verify enabled; see: https://walletconnect.slack.com/archives/C04DB2EAHE3/p1708533305840129?thread_ts=1708529093.070159&cid=C04DB2EAHE3

Solution

We should document known common scenarios like this where a dapp developer will see errors like this and provide them with guidance.