[docs]: add docs for setDomainPolicy

[seanmcguire12]

why

what changed

test plan

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Summary by cubic

Adds documentation for context.setDomainPolicy() and context.getDomainPolicy() to help users control HTTP(S) requests by domain across a context, with clear examples and error details. Addresses Linear STG-2426.

• New Features
  • Documented setDomainPolicy(policy: DomainPolicy | null) and getDomainPolicy() with TypeScript signatures and a new “Domain Policy” example tab using @browserbasehq/stagehand.
  • Clarified domain-only patterns, wildcard behavior, precedence (blockedDomains over allowedDomains), scope (pages, iframes, subresources), non-HTTP scheme behavior, and how to clear the policy.
  • Updated API reference with related errors (StagehandInvalidArgumentError, StagehandSetDomainPolicyError) and V3Context/DomainPolicy interface snippets.

^{Written for commit 4468c7c. Summary will update on new commits.}

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Latest commit: 4468c7c

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