Document LuigiElement feature on docu website

[ndricimrr]

We've had this feature there, and working, but in the docu we don't show it apparently. At least it doesn't appear on any .md file in the repo so I don't think Indexing is at fault.

Since there are people probably using it might be worth documenting it for other potential users.

Maybe give out a simple example for how to use it and what benefits there are from using it.

Find out if there were reasons not to document it in the first place and if none found then discuss where and how to put it in the docu website.