Add an experience rating for tutorials

The `/tutorials/index` page enumerates all tutorials available in `/tutorials`. Many of these involve scripting. But it would be nicer if each page has an experience rating in the front matter, which could be shown next to the tutorial, so that readers can pick tutorials of interest to them. Is it about using/learning ImageJ? Extending it? Or developing for it?

Start with `/tutorials/index` and the tutorial pages under `/tutorials`, reviewing how their front matter is structured. Define how the experience categories should be represented and displayed beside each tutorial. Done means tutorials have an experience rating in front matter and the index page shows it for readers.

Written by the indexing model from the issue text.