Update from code changes: Cobalt PolicyRegistry composite policies

[mintlify]

Summary

Add a Cobalt reference page documenting the new UNION / INTERSECT composite policies on PolicyRegistry.

Changes

• New page base-chain/specs/upgrades/cobalt/policy-registry-composite-policies.mdx covering the UNION = 2 / INTERSECT = 3 PolicyType values, the new createCompositePolicy, updateComposite, compositePolicyChildIds, and MIN_COMPOSITE_CHILD_POLICIES / MAX_COMPOSITE_CHILD_POLICIES selectors, the CompositePolicyUpdated event, revert precedence for both write paths, and the new IncompatiblePolicyType revert on createPolicy / createPolicyWithAccounts for composite policyType values.
• Call out that Cobalt is not yet live and every composite selector is undialable until activation.
• Wire the new page into the Cobalt group in docs.json.

Context

Mirrors the base-std Cobalt changelog entry added in base/base-std#200.

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