Update and restructure documentation

[gamatos]

• Restructure contents by grouping statetensor, utility functions
• Add all_zeros_statetensor, all_zeros_densitytensor to documentation
• Add repeat_circuit to documentation

[cqc-melf]

Have you checked if the documentation still builds correctly?

[gamatos]

Have you checked if the documentation still builds correctly?

Seems to build fine for me locally!

[gamatos]

Awesome!