Addons: add documentation for CustomScript

[humitos]

We need to document this addon and put some examples on how to solve common issues with this feature.

• Refs: Adddons: allow injecting an "always live" JavaScript file #11758
• Requires: Addons: serve the file from Read the Docs for CustomScript #11774
• Requires: Addons: add UI for CustomScript ext-theme#527
• Requires: API: save a "flag" on window object addons#474

[jeyasimma]

Please assign it to me, I am trying to add the documentation for CustomScript.

[ericholscher]

We actually aren't exposing this to users -- should we, and write docs for it?

[humitos]

Yeah. I think it's fine to have small docs for this addon. It's not polished/perfect, but it's useful. We can add the other features mentioned in the description of the issue in the following iteration when more people are using it and requesting for upgrades.