docs: document the event loop

[tgiachi]

Docs

Documents the EventLoopService shipped in v0.14.0.

• Scheduler article — new Event loop section: what it does (drains the dispatcher + advances the timer wheel each frame), RegisterEventLoop(), the eventLoop YAML config, metrics (eventloop provider), and the event loop vs. timer-wheel pump mutual exclusivity via ITimerWheelDriver.
• Events/jobs/scheduling tutorial — note on the timer-wheel driver choice (pump vs. event loop).
• Services.Core README — EventLoopService added to the Key types table.

API reference pages are regenerated by DocFX from the XML docs on the new public types. Merging to main rebuilds and publishes the site.