[FIX] UI: Improve documentation about plugin manipulations

[thibsy]

Hi folks,

This PR improves the documentation about manipulations plugins can make to the UI framework. Primarily, I ...

• added some insight about the rendering chain plugin developers need to worry about, because I have stumbled over this several times myself now :).
• added some documentation about introducing custom UI components, because I think this is a powerful functionality which needs some more recognition, along with a code example.
• improved the existing example because it featured some wrong comments and lacked detail.
• improved the document grammatically.
• removed the contribution section, because we have VCS for this.

P.S. Unsure about the proper ILIAS copyright license headers in examples.

Kind regards, @thibsy