Minimal Rust project example for getting Latex in docs

This approach use a HTML header loading a script in the final HTML doc pages. You need to set environmental variable RUSTDOCFLAGS with paramenter html-in-header to a fragment of HTML file with the script that load and configure Katex.

The html file is something like this

```html

```

In this minimal example repo is in src/docs-header.html.

And you put Latex in your docs like

```rust /// $$E = mc^2 $$ /// $$m = \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}$$

```

For compiling the docs:

NIX kind

sh RUSTDOCFLAGS="--html-in-header src/docs-header.html" cargo doc --no-deps --open

Windows

cmd

set RUSTDOCFLAGS=--html-in-header src\docs-header.html cargo doc --no-deps --open

PowerShell

$env:RUSTDOCFLAGS="--html-in-header .\src\docs-header.html" cargo doc --no-deps --open