Migrate third party plugins guidelines to the book (#11242)

# Objective

- Describe the objective or issue this PR addresses.
- If you're fixing a specific issue, say "Fixes #X".

Removes the old third party plugin guidelines section and related links
since it is being moved to the Bevy book in the site per
bevyengine/bevy-website#848 and
bevyengine/bevy-website#298

## Solution

- Describe the solution used to achieve the objective above. 

Removes the old links and files.

README.md

@@ -92,12 +92,6 @@ This [list][cargo_features] outlines the different cargo features supported by B
 
 [cargo_features]: docs/cargo_features.md
 
-## [Third Party Plugins][plugin_guidelines]
-
-Plugins are very welcome to extend Bevy's features. [Guidelines][plugin_guidelines] are available to help integration and usage.
-
-[plugin_guidelines]: docs/plugins_guidelines.md
-
 ## Thanks
 
 Bevy is the result of the hard work of many people. A huge thanks to all Bevy contributors, the many open source projects that have come before us, the [Rust gamedev ecosystem](https://arewegameyet.rs/), and the many libraries we build on.