[RF-DOCS][ci skip] Update Threading and Code Execution Guide (rails#55179)

This PR has been created to update the Threading and Code Execution Guide as part of the Rails Documentation project.

### Detail

The following changes have been made:

- Phrasing and language choices have been updated for improved clarity and simplicity.
- The structure of the guide has been slightly altered to improve flow
- Information about asynchronous Active Record has been added. 
- The 'work in progress' label has been removed.
- Examples of wrapped application code have been added.
- Information on Isolated Execution State has been added.
- A section on `CurrentAttributes` has been added.

Author: OughtPuts

guides/source/documents.yaml

@@ -296,7 +296,6 @@
       name: Threading and Code Execution in Rails
       url: threading_and_code_execution.html
       description: This guide describes the considerations needed and tools available when working directly with concurrency in a Rails application.
-      work_in_progress: true
 -
   name: Contributing
   documents: