docs: clarify packaged runtime plugin compatibility

[chaiyuntian]

Custom .pak users currently have no runtime-plugin compatibility guidance in the packaging tutorial. Add the support information confirmed by the maintainer in issue #80, dated February 15, 2026: Control Rig / Full Body IK / IK Rig and Chaos Cloth are enabled in the build discussed there. Explicitly leave ML Deformer and Physical Animation unconfirmed rather than inferring support.

Explain the distinction between content packaging and executable runtime modules, and fix the tutorial's two broken Additional Environments links.

Refs #80. This documents the existing confirmation; it does not enable plugins, assert a complete manifest for later releases, or close the remaining unconfirmed capabilities.

Validation evidence

• Sphinx 7.1.2 HTML builds successfully on Python 3.10.
• Compared against unmodified 1f588ec: 3 warnings before, 1 after. Both repaired additional_environments references now resolve; the remaining warning is the existing unrelated components/citygen.md:135 reference to unrealcv+.md.
• Parsed the generated HTML to verify the support matrix, dated primary-source link, and both repaired links and their target file.
• git diff --check passes.

Follow-up runtime verification: launched the complete official Base20260201 Windows package (ZIP SHA-256 b737c33da55a068f142974d28dd83dfd1f32de86800ee469c312653c55810e5f). Its UE 5.3.2 startup log records mounting ControlRig, FullBodyIK, IKRig, and ChaosCloth, consistent with the documented maintainer confirmation. Committed engine log excerpt and package/reproduction details.

This confirms plugin mounting in that Windows release. Custom content packaging, ML Deformer, and Physical Animation remain unverified; mounting DeformerGraph is not evidence of ML Deformer support.