docs: use `pytket-docs-theming` submodule (#84) #344

Job	Run time
cirq - Build and test module (ubuntu-22.04)	2m 23s
cirq - Build and test module (macos-12)	3m 48s
cirq - Build and test module (windows-2022)	8m 22s
Publish to pypi	0s
Build docs	0s
	14m 33s

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