docs(idioms): add functional programming and borrowed types patterns

Add comprehensive Vietnamese guides for Rust idioms:
- functional-programming.md (481 lines): FP concepts in Rust including immutability,
  higher-order functions, iterators, combinators, and pattern matching
- borrowed-types.md (511 lines): Using borrowed types for function arguments
  (&str vs String, &[T] vs Vec<T>, &Path vs PathBuf) with performance benefits

Includes practical examples, comparisons with imperative style,
and best practices for idiomatic Rust code.

Co-Authored-By: Claude <[email protected]>

Author: claude