Document legacy AMI opt-in and ASUS EXTRNIP download_endpoint

[nesvet]

Summary

Documentation only — legacy AMI opt-in pointers and ASUS EXTRNIP download_endpoint notes for BMCs without a DNS hostname.

Complements #37 when implementation and deployment docs land in separate pull requests. This PR does not duplicate the opt-in flags table from #37.

Contents

• When to enable allow_legacy_jar_signatures / allow_insecure_jnlp_certs (see Opt-in legacy AMI JAR signatures for sandboxed KVM child #37 for the full table)
• EXTRNIP placeholder behaviour for JNLP download URLs
• Example ASMB8-iKVM host template:

hosts:
  my-bmc:
    full_hostname: 10.0.0.9
    download_endpoint: "Java/jviewer.jnlp?EXTRNIP=10.0.0.9&JNLPSTR=JViewer"
    allow_legacy_jar_signatures: true
    allow_insecure_jnlp_certs: true

Tested on ASUS ASMB8-iKVM firmware 1.14.2.