Add a button-controlled pause and resume example

[devkyato]

Create a beginner-friendly Arduino IDE example that combines LedAnimator with a debounced push button, without adding blocking delays to the library.

Acceptance criteria

• new example appears under examples/ButtonPause/
• uses a simple millis()-based debounce in the sketch
• pauses and resumes a repeating animation
• includes wiring comments and works with active-low INPUT_PULLUP
• compiles in the Uno CI environment
• no new library dependency is introduced

This is intentionally an example-level contribution so students can practice non-blocking state handling before proposing a new public API.