Add support for MITRE ATT&CK

extern/README.md

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+# External data
+
+Some data is required for either core functions or addons.
+
+## MITRE ATT&CK data
+
+The attacks described in the MITRE ATT&CK framework is available on GitHub: [mitre-attack/attack-data-model](https://github.com/mitre-attack/attack-data-model).
+
+It is filtered to only required values using this [nushell](https://www.nushell.sh/) one-liner:
+```nu
+http get https://raw.githubusercontent.com/mitre-attack/attack-stix-data/refs/heads/master/enterprise-attack/enterprise-attack-18.1.json | get objects | where type == "attack-pattern" | each {|e| let ref = ($e.external_references | where source_name == 'mitre-attack' | first | get external_id url); {name:$e.name, id: ($ref | first), url: ($ref | last) } } | to json | save  mitre-attack.json
+```
+
+The filtered output is saved to `mitre-attack.json` and used by `addons/mitre.typ`.
+
+### License
+
+> The MITRE Corporation (MITRE) hereby grants you a non-exclusive, royalty-free license to use ATT&CK® for research, development, and commercial purposes. Any copy you make for such purposes is authorized provided that you reproduce MITRE's copyright designation and this license in any such copy.
+> "© 2025 The MITRE Corporation. This work is reproduced and distributed with the permission of The MITRE Corporation."
+
+For more information, see [MITRE's Terms of Use](https://attack.mitre.org/resources/legal-and-branding/terms-of-use/).