Centralize policies

[stkent]

As discussed here, create a policy page that collects all track policies. The page should take the form of a checklist that maintainers can use (a) when onboarding, to familiarize themselves with track policies, and (b) as part of periodic or change-driven maintenance (e.g. during PR review). A suggested format is below, but feel free to discuss and iterate on it!

Policy topic	Policy description
Multiple file submissions	The first exercise in the track whose test suite mandates multiple solution files should be accompanied by a HINT.md file reminding the practitioner that the CLI supports multiple file submissions.

Event	Topics to review
The track order changes	Multiple file submissions, ...
The number of solution files required by an exercise test suite changes	Multiple file submissions, ...