Add prove_threshold_policy to Identity section

[cypriansakwa]

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Checklist

• The URL is not already present in the list (check with CTRL/CMD+F in the raw markdown file).
• Each description starts with an uppercase character and ends with a period.
  Example: solc-js - JavaScript bindings for the compiler.
• Drop all A / An prefixes at the start of the description.
• Avoid using the word Solidity in the description.