doc: consider re-adding python-tuf attack protection demo

[lukpueh]

Description of issue or feature request:
#1790 removes severely outdated ATTACKS.md, which demoed how python-tuf protects against the attacks TUF is designed for.

Such documentation, if up-to-date, still seems like a good source for better understanding the purpose and necessity of TUF and thus should be re-added.

Current behavior:

• no attack protection demo

Expected behavior:

• add attack protection demo for ngclient
  (potential source of inspiration: ngtests: ensure tests cover known attacks on package managers #1640)

[lukpueh]

Adding suggestion from duplicate closed #360:

This metadata [required to demonstrate the attacks] can be fed to other implementations to verify that they correctly handle these attacks