Docs: Add example on how to make a custom sorting indicator component

[MichalBryxi]

There is a section on sorting and technical API doc on ember-th/sort-indicator.

From the composability perspective and utility frameworks (like tailwindcss) I think it would be good to have an example on how to provide custom sorting indicator component.

I made a quick post on this topic to help the community. If I get a thumbs up and a hint which part of the documentation would be best to put this information into, I can create a PR with the same example as in the article above.