Add illustration of ∂self in the maths/propagators section

[ablaom]

In response to a suggestion at this Julia discourse thread.

Actually, there is an existing example in the docs, the "Pedagogical Example". I expect it's still worth having another, simpler example, in the more specific context addressed here. At least, this would have helped me.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (26b7748) 93.90% compared to head (dc7bf6b) 93.90%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #640   +/-   ##
=======================================
  Coverage   93.90%   93.90%           
=======================================
  Files          15       15           
  Lines         903      903           
=======================================
  Hits          848      848           
  Misses         55       55           

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[oxinabox]

Thanks!