Documents view angle struct and related API

Adds documentation for the `omath::ViewAngles` struct,
clarifying its purpose, common usage patterns,
and the definition of the types of pitch, yaw and roll.

Also, adds short explanations of how to use ViewAngles and what tradeoffs exist
between using raw float types and strongly typed Angle<> types.

docs/linear_algebra/mat.md

@@ -1,4 +1,4 @@
-# `omath::Mat`
+# `omath::Mat` — Matrix class (C++20/23)
 
 > Header: your project’s `mat.hpp` (requires `vector3.hpp`)
 > Namespace: `omath`