Quick Start Guide Writing Adjustments Post-Book Launch

Several pages (or sections within some pages) in the Quick Start Guide are either out of date or are better presented in the Bevy Book. As well, the Bevy Book itself should be referenced in several places in the Quick Start Guide once it is published. This issue will be an overview of the writing that needs to be updated, replaced, or outright removed once the Bevy Book has been published.

- [Introduction Page](https://github.com/bevyengine/bevy-website/blob/main/content/learn/quick-start/introduction.md)
- Should mention the Bevy Book and the differences between the Book and the Quick Start Guide.

- [Setup Page](https://github.com/bevyengine/bevy-website/blob/main/content/learn/quick-start/getting-started/setup.md)
- Remove `Compile with Performance Optimizations` section.
- Remove `Enable Fast Compiles` section.
- Remove `Improve Runtime Performance` section.
- Replace all of the above with links to the relevant Book pages?
- Link to relevant Book chapters in the appropriate sections.

- [Next Steps Page](https://github.com/bevyengine/bevy-website/blob/main/content/learn/quick-start/next-steps.md)
- Mention and link to the Bevy Book.

- [Building Bevy's Ecosystem Page](https://github.com/bevyengine/bevy-website/blob/main/content/learn/quick-start/plugin-development.md)
- Remove page altogether.
- Maybe briefly mention plugin development in the [Plugin Page](https://github.com/bevyengine/bevy-website/blob/main/content/learn/quick-start/getting-started/plugins.md) and link to the relevant Book pages?

Review the listed Quick Start Guide files: introduction.md, getting-started/setup.md, next-steps.md, plugin-development.md, and getting-started/plugins.md. Compare their current content with the published Bevy Book, then update or remove the specified sections and pages so the Quick Start Guide links to the relevant Book material and explains the distinction between them.

Written by the indexing model from the issue text.