docs: lead the Secrets guide with V4 bindings

[MagMueller]

Lead the Secrets guide with Python and TypeScript examples for V4 inline bindings, and keep the existing V2 recipe under a legacy heading. Explain run scope, hostname limits and the boundary after typing: a page or agent with CDP access can read the value. Synchronize the four generated guide/index copies.

Executed both exact examples against the current SDK request builders with synthetic credentials and no provider calls, checked the current V4 schema, regenerated the docs and checked links. No hosted login or end-to-end security claim.

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Summary by cubic

Leads the Secrets guide with Python and TypeScript examples for API V4 inline secret bindings, and moves the existing V2 recipe under a legacy heading.

• Documents run-scoped bindings with alias, inline source, and allowedDomains hostname limits.
• Clarifies that typed secret values are accessible to the page and to agents with browser/CDP access.
• Synchronizes the four generated guide copies (llms.txt, llms-full.txt).

^{Written for commit b8d665b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 5 files

_{Re-trigger cubic}