Remove duplicate navbar and sphinx configuration

[CalMacCQ]

We should be able to move the nav-config.js to the repository that builds the website. This saves having duplicate config that needs to be updated.

[CalMacCQ]

Working towards a solution for this. The pytket-docs-theming repository contains a source of truth for navbar, quantinuum-sphinx version and sphinx config. Can share these files by using pytket-docs-theming as a submodule.

This has the additonal benefits of making local build much easier and removing some legacy boilerplate code from the extensions build.

DEMO PR -> CQCL/pytket-cutensornet#153