The Game Loop Book Chapter Refresh Tracking Issue

[The Game Loop Book chapter](https://bevy.org/learn/book/the-game-loop/) is sorely needing an update, especially compared to other chapters which have been reworked or updated over the last few months. This issue is meant to help organize the content that we want to see place into this chapter.

**New Content**

**Existing Content Updates**
- [App](https://bevy.org/learn/book/the-game-loop/app/)
- Expand information and context of what a Bevy App is and can be used for.
- Render App could be a great example.
- [Schedules](https://bevy.org/learn/book/the-game-loop/schedules/)
- Expand Fixed schedules explanations & explain where to use them (#2445)
- [Time](https://bevy.org/learn/book/the-game-loop/time-and-timers/)
- Move Timers and Stopwatch to Control Flow (#2427)
- Explain Real, Virtual, Fixed, and Time resources.

Start with the linked Game Loop chapter and its App, Schedules, and Time pages, then review referenced issues #2445 and #2427. Done means the chapter content is refreshed, Fixed schedules and time resources are explained, and the listed Timer and Stopwatch update is addressed.

Written by the indexing model from the issue text.