docs: Rewrite and enhance 'wait' command chapter

[TANAY-BARGIR]

Addresses #190.

This PR is a complete rewrite of the wait command chapter to improve clarity and add more detail.

• The introduction now explains background processes.
• A new example for waiting on all child processes has been added.
• The original example now has a detailed explanation of the script's components.

Thanks!

[TANAY-BARGIR]

Hi there! It looks like the automated build check failed. I checked the log, and it seems to be a project-wide issue related to a missing asset file (ebook/en/assets/highlight.codeblock.min.css).

My documentation changes are ready for review whenever you get the build system sorted out. Happy to help if you need anything from my end!