Add VBlank interrupt lesson

[renatomoraesp]

What changed

• Added a new Part II lesson, "VBlank Interrupts".
• Added an Unbricked example that uses a VBlank interrupt handler at $0040.
• Replaced the main-loop scanline busy-wait with a WaitForVBlank routine that sleeps with halt until the VBlank interrupt sets a flag.

Fixes #157

Testing

• git diff --check
• rg -n "[ \\t]+$" src/SUMMARY.md src/part2/vblank-interrupts.md unbricked/vblank-interrupts/main.asm unbricked/vblank-interrupts/build.sh unbricked/vblank-interrupts/hardware.inc
• docker run --rm --platform linux/amd64 --mount "type=bind,source=$(pwd),target=/workspace" -w /workspace/unbricked/vblank-interrupts ghcr.io/gbdev/rgbds:v1.0.1 bash build.sh
• Ran the project mdBook build in Docker. The custom and HTML backends rendered the new page; the final optional linkcheck backend failed because the project image did not include mdbook-linkcheck.

Notes

This introduces interrupt-based frame timing but does not add shadow OAM or OAM DMA yet; the lesson calls that out as a follow-up step.

The implementation was AI-assisted and human-reviewed.