Document competitor match arrivals process

[RealOrangeOne]

We keep re-inventing this every year - we shouldn't. The basics mostly don't change, so we should document them so we don't need to re-invent it.

Related resources:

• https://github.com/roboticsoutreach/smallpeice-handbook/blob/main/docs/competition/match-processes.md
• SR2025 welcome talk slides