Fix broken links to API docs

[tcNickolas]

@"" style of links doesn't seem to work if the reference is in lowercase.
Includes fixing broken formulas in PurifiedMixedState.

[tcNickolas]

The formulas at API page for https://docs.microsoft.com/en-us/qsharp/api/qsharp/microsoft.quantum.preparation.purifiedmixedstatewithdata are also broken, but I couldn't figure out what caused it at a glance.

[cgranade]

@"" style of links doesn't seem to work if the reference is in lowercase.

That's right, I don't think that ever worked properly; I had thought I fixed most of that with the new docgen tool, but thanks for catching this one as well. For the most part, <xref:Fully.Qualified.Name> seems to work most reliably, but @"Fully.Qualified.Name" should work as well.

Includes fixing broken formulas in PurifiedMixedState.

Thanks for catching that! Wish we had a better way of previewing rendering... ah, well.

[cgranade]

The formulas at API page for https://docs.microsoft.com/en-us/qsharp/api/qsharp/microsoft.quantum.preparation.purifiedmixedstatewithdata are also broken, but I couldn't figure out what caused it at a glance.

I'll take a look; that used to work, but I'm guessing there was a change to how Markdig escapes things before they get to MathJax.