Add documentation for [pipx.run] entry point

[matthewfeickert]

TODO: Write documentation on this.

Originally posted by @uranusjr in #615 (comment)

I only learned about the pipx.run entry point earlier today by asking about it and getting shown how build uses it, but it would be quite useful to have some docs on this.

[chrysle]

I must admit I also wasn't aware of that entry point. Thanks for raising this issue!