Add build time optimization documentation (#3608)

This document describes techniques for reducing C++ template instantiation
overhead in the Composable Kernel codebase, including:

- Replacing recursive templates with pack expansion (O(N) → O(1) depth)
- Using named functors instead of lambdas to share instantiations
- Replacing template recursion with constexpr loops
- Using fold expressions for accumulation operations

These techniques can significantly reduce build times for template-heavy code.