Blueprint: `\discussion` macro

Stream: Equational

[Pietro Monticone]

The LeanBlueprint library features another macro called \discussion{} which takes GitHub issue number where the surrounding definition or theorem statement is discussed.

I believe that it can be used more systematically now by who writes the Unclaimed Outstanding Tasks (@Terence Tao in this case).

It will appear both in the blueprint website and the related dependency graph.

Readers might be interested to now about the underlying technical discussion, inspect the changes in the related PR, etc.