docs: document TypeScript onSchedule reducer/procedure registration

[cloutiertyler]

Description of Changes

#5435 added the onSchedule option to reducer() and procedure() in the TypeScript bindings, but it was never documented. This PR documents it in the current (next) docs:

• Schedule Tables (00500-schedule-tables.md): the TypeScript tab now leads with onSchedule as the recommended registration form, explains that it removes the table→reducer forward reference (so the table and its scheduled function can live in separate files without an import cycle), notes the at-most-one-scheduled-function-per-table constraint, and demotes the legacy scheduled: (): any => reducerName table option to a legacy callout.
• Scheduling Procedures (00300-reducers.md): example switched to procedure({ onSchedule }, params, t.unit(), fn), with a note that a scheduled procedure must return t.unit().
• Cheat sheet and Reducer Context: TypeScript examples updated to the new syntax.

migrating-to-2.0.md intentionally keeps the legacy syntax since it documents the historical migration path.

Testing

• pnpm build in docs/ succeeds

Closes nothing; follow-up documentation for #5435.

[clockwork-labs-bot]

Docs review: no blocking findings.

The new examples match the TypeScript implementation: reducer({ onSchedule }, params, fn) and procedure({ onSchedule }, params, t.unit(), fn) are supported, and the existing TS tests cover the relevant schedule cases, including duplicate schedule bindings, missing ScheduleAt, and the scheduled procedure t.unit() return constraint.

I also verified locally on the PR head that pnpm build succeeds from docs/ after installing dependencies in a temporary worktree.

One non-blocking docs nit: the lead TypeScript example in schedule-tables.md now recommends onSchedule, but the snippet still starts after spacetimedb would have been defined. Adding the imports plus const spacetimedb = schema({ reminder }); export default spacetimedb; would make the recommended example copy/pasteable. This was already true before this PR, so I would not block on it.

[cloutiertyler]

Addressed the docs nit in ef42cfc: the lead onSchedule example in schedule-tables.md now includes the imports and const spacetimedb = schema({ reminder }); export default spacetimedb; setup, so it's copy/pasteable as-is. Docs build verified passing.

[JasonAtClockwork]

LGTM