Doc Revision: Client SDK pages should reference CORS setup

[tomkis]

Summary of Requested Changes:
The custom UI client SDK docs (a2a-client.mdx, platform-api-client.mdx, getting-started.mdx) don't mention CORS configuration. Developers building custom UIs will hit CORS errors when connecting to the server from a browser. These pages should reference the existing CORS configuration section in the deployment guide.

Requirements

• Client SDK docs mention that CORS must be enabled on the server for browser-based clients
• Link to the existing CORS section in the deployment guide

Task Checklist

• Contributing Guideline Compliance: I read and complied by "Documentation Contribution Guidelines" section of the Agent Stack project CONTRIBUTING.md guide.
• Snippet Test: Verified all code works in a fresh environment (fresh agent stack install and clean environment).
• Visual Preview: Checked formatting/layout via Mintlify.
• CLI Sync: Ran mise run agentstack-cli:docs (if applicable).
• Location: Updates placed in docs/development for go live during next release or both docs/development and docs/stable for urgent go live.

Related: #2015