Document the --health command

[EricCrosson]

PR #1669 introduced the hx --health command, which I used tonight to help a friend diagnose LSP on his machine. It works to great effect! But I had to look at the PR to find the following information about the color-coded output:

(red for not found, green for found, yellow for no configuration)

and noticed the --health command isn't documented on the web documentation anywhere.

I took a quick look to see if I could add this documentation myself but couldn't come up with a strong opinion on where it should live.

[sudormrfbin]

We could add a separate troubleshooting section and mention it there, with a link to it from the installation section. It's also possible to run hx --health LANG to see diagnostics for a particular language, whereas hx --health is meant to give a bird's eye view of everything.

[EricCrosson]

Closed by #2357 -- thanks @RobWalt!