docs: Rework the Scheduling section for 4.0

[developerjamiu]

Summary

Section-by-section rework of the Scheduling (future calls) concept pages for Serverpod 4.0, following the same pattern as the Data & the database (#696) and Web server (#711) passes. Adds a section overview and brings every page current with the 4.0 API, verified against the framework source.

Changes

• New Overview page. The section now opens with a landing page: what a future call is, the three scheduling shapes, the at-least-once execution guarantee, and the run-mode facts. Inbound links retarget here.
• Future calls (was Setup): explains the generated .example accessor, uses the standard serverpod start / serverpod generate framing, and corrects the UTC behavior of callAtTime.
• Recurring tasks (rewritten): documents the native callRecurring().cron() / .every() API, which was previously undocumented. Cron schedules are noted as UTC. The manual self-reschedule pattern is kept as a short fallback for dynamically computed intervals.
• Inheritance (reframed): centers the real use case of a module exposing future calls through an abstract base, and fixes an example that claimed to share behavior through super.
• Configuration: options table, corrected concurrency semantics (0 also means unlimited), and a corrected note that --role maintenance executes due calls rather than only checking for broken ones.
• Legacy: states the @Deprecated status of the string-based scheduling methods precisely.

Notes

• Execution is at-least-once (not exactly-once); the previous wording overstated the guarantee.
• All framework claims verified against 4.0.0-beta.0.

[Zfinix]

The two renamed URLs here need redirects too, same list as the testing PR.

[Zfinix]

LGTM 🚀 approved when the apostrophe is fixed.