Add instructions on `#include` style to our style guide

We're not totally consistent with how we include headers. What about something like this?

* Use quotation marks for header files in the same directory e.g. `#include "Library.h"`
* Use angle braces for everything else e.g. `#include `

It would be great if clang-format could enforce this. In any case, once we agree on something it should go in the [C++ Style Guide](https://github.com/CesiumGS/cesium-native/blob/main/doc/style-guide.md).

Read the discussion on the issue first to determine whether the proposed include rule has been accepted. Then inspect doc/style-guide.md and the repository's existing clang-format setup. Done means the agreed guidance is documented in the C++ Style Guide and formatter enforcement is addressed if the project supports it.

Written by the indexing model from the issue text.