Crate katex_doc

Utilizing the html injection feature of rustdoc, we can enable the use of $\KaTeX$ in documentation!

For inline math, use $`1+1`$, which renders as $1+1$.

For display math, use:

```math
f(x) = \int_{-\infty}^\infty
  \hat f(\xi)\,e^{2 \pi i \xi x}
  \,d\xi
```

(example taken from https://khan.github.io/KaTeX/), which renders as:

f(x) = \int_{-\infty}^\infty
  \hat f(\xi)\,e^{2 \pi i \xi x}
  \,d\xi

To enable this on docs.rs, you need to add the following to your Cargo.toml:

[package.metadata.docs.rs]
rustdoc-args = [
    "--html-in-header",
    ".cargo/registry/src/github.com-???/katex-doc-0.0.0/katex.html",
]