Add documentation for [pipx.run] entry point

[matthewfeickert]

TODO: Write documentation on this.

Originally posted by @uranusjr in https://github.com/pypa/pipx/issues/615#issuecomment-769569226

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!

[jaraco]

Looks like the implementation is here:

https://github.com/pypa/pipx/blob/dc9f9a849a918c68bc2cc99c72651022371fcf9f/src/pipx/venv.py#L405-L407

~~It also looks like it doesn't play well with local installs.~~ (edit: wasn't correct on latest code)

[chrysle]

~It also looks like it doesn't play well with local installs.~ (edit: wasn't correct on latest code)

Yes, I encountered this issue myself when trying out the entry point. The fix was included in version 1.6.0.

FYI mentioned this in my proposal for a new PyPUG guide on packaging command-line applications at pypa/packaging.python.org#1553; if anyone would care to take a look, that would be greatly appreciated!