docs: document poly chat --sip-header on the ADK CLI reference

[polyai-moaed]

What

Ports the poly chat --sip-header documentation from the ADK repo across to the Mintlify ADK tab, so the option is documented on docs.poly.ai and not only on the MkDocs site.

--sip-header NAME=VALUE lets a developer simulate SIP headers when starting a chat conversation, which exercises agent behaviour that reads conv.sip_headers without arranging a real SIP call.

Dependency

Documents the option added in polyai/adk#256, which is open (all checks green). That PR in turn depends on PolyAI-LDN/poly_core#44668 for the chat API to accept the headers. Worth holding this until adk#256 merges so the docs do not describe an unreleased flag.

Changes

adk/reference/cli.mdx

• poly chat examples block: two --sip-header invocations, single and repeated.
• New #### Simulating SIP headers subsection under poly chat, covering the NAME=VALUE form, repeating the flag for multiple headers, exposure to project functions via conv.sip_headers, and the two limits (it does not create a SIP call or reproduce carrier behaviour, and headers cannot be changed when resuming a conversation).
• poly chat flags summary table: a --sip-header NAME=VALUE row.

Wording is carried over from the upstream docs/docs/reference/cli.md so the two sites stay in step. No docs.json change is needed, adk/reference/cli is already in the nav.

Verification

• mint validate: build validation passed.
• mint broken-links: 3 broken links found, all pre-existing and all the same /managed-topics/how-to-setup-action/handoff target in integrations/chat/amazon-connect.mdx, nice-cxone.mdx, and webex.mdx. None are in the file this PR touches.

Notes

• Ported by hand rather than by re-running .migration/convert.py. A full rm -rf adk regeneration would revert the hand edits on adk/index.mdx from docs: wide mode and linked recommended path on ADK overview #581 (mode: "wide") and docs: redesign home landing page + ADK GitHub link #586 (the "View the ADK on GitHub" card), since neither exists in the MkDocs source. Worth folding those into convert.py before anyone runs a full resync.
• Related drift, not in scope here: poly audio-cache (polyai/adk#246, merged 4 Aug) is documented upstream with 7 subcommands and is still absent from adk/reference/cli.mdx.