docs: add example notebooks into documentation pages

[CalMacCQ]

Description

Adding the notebook examples into the docs html pages. The coversion to html is done by the MyST-NB library. I'm avoiding execution for now. To show the cell outputs would require setting up GPUs in C.I. in the repository that builds the website or somehow executing them in this repository and just deploying the html from the website repository.

remaining TODOs

• validate file path hack to avoid executing notebooks (working locally) -> fix: avoid executing pytket-cutensornet notebooks pytket-docs-theming#8
• fix C.I. failures (fixed check-examples, just need the nix c.i. fixed.)

Related issues

closes #154

Checklist

• I have performed a self-review of my code.
• I have commented hard-to-understand parts of my code.
• I have made corresponding changes to the public API documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the changelog with any user-facing changes.

[PabloAndresCQ]

Looks great and works locally (as long as the root folder is named pytket-cutensornet :P)