Clean up Notebooks repository

[CalMacCQ]

Diff is a bit inflated due to adding a new notebook and deleting an old one. Also deleted a big JSON file which was unused.

Cleaned up several examples

Fixed some dodgy maths formatting in the 3 pytket-qujax examples and my new phase estimation notebook

Phase estimation (current version)

A qujax example with dodgy formatting -> https://tket.quantinuum.com/examples/pytket-qujax-classification.html

deleted the tket_benchmarking example this is TOTALLY legacy as it says in the notebook itself
https://tket.quantinuum.com/examples/tket_benchmarking.html

Replaced the single letter variable names in conditional_gates_example.

Imporved markdown headings

Added categories of examples (See image)

[isobelhooper]

I think this looks good for structural cleanup!

Would like to do another pass through it for spelling / punctuation / grammar, but I think it'd be reasonable to do that in a separate PR.