docs: Add section on angles

[tatiana-s]

Closes #16

[tatiana-s]

I think it would probably be better to have a more interesting example than this which basically just does what the FAQ does but couldn't think of a good one

[CalMacCQ]

This looks good to me. I'd like to add more but I'm not sure what more we would need to explain. Could be worth getting a second opinion. Perhaps from @cqc-alec ?

[cqc-alec]

This looks good to me. I'd like to add more but I'm not sure what more we would need to explain. Could be worth getting a second opinion. Perhaps from @cqc-alec ?

It could be worth mentioning why we have this type, as it may not be obvious. Namely, it allows exact representation of angles for gates in the Clifford hierarchy. This can be useful for optimization.

[CalMacCQ]

It could be worth mentioning why we have this type, as it may not be obvious. Namely, it allows exact representation of angles for gates in the Clifford hierarchy. This can be useful for optimization.

That is a good point, thanks.