Improve CI/CD script documentation

What does this MR do?

I started to work on a copy edit to improve CI YAML doc, focusing on "ing" words, based on the related issue below. During this edit, I realized there was a long overdue change I needed to make to the script documentation (see other issue):

• before_script and after_script needed to be split into their own sections. They are keywords on their own, and they work differently, so they should be documented separately. This makes the examples clearer as well.
• "tricks" to get code coloring and multi-line commands in script sections should be moved into their own docs, as they aren't really explaining how to use the script CI/CD keywords themselves. This will allow more extensive examples in the new doc as well, as some of the details are different for different shells/OSes.
• The YAML anchors for the different types of scripts can all be combined into one.

Related issues

• Related to #238886 (closed)
• Closes #39490 (closed)

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