remove Isabelle walkthrough example

[HuStmpHrrr]

https://proofgeneral.github.io/doc/master/userman/Basic-Script-Management/#Walkthrough-example-in-Isabelle

it seems Isabelle is in the doc. I don't suppose Isabelle is maintained anymore?

[erikmd]

@HuStmpHrrr good catch!

Rather than just removing this part, I guess it'd be nicer to somewhat "translate" it to, say, Coq (?)