Auto merge of #6271 - nholden:bundle-doctor-man-page, r=colby-swandale
Auto merge of #6271 - nholden:bundle-doctor-man-page, r=colby-swandale

Add man page for `bundle doctor`

### What was the end-user problem that led to this PR?

The `bundle doctor` command was missing a man page (see https://github.com/bundler/bundler/issues/6243).

### What was your diagnosis of the problem?

Running `bundle help doctor` from the command line showed [the description from `cli.rb`](https://github.com/bundler/bundler/blob/723608f45866cee0f1b315551a8dde6a99efebc6/lib/bundler/cli.rb#L611-L620), but it wasn't displayed in the same format as commands with man pages. Documentation for `bundle doctor` was also missing from http://bundler.io/docs.

### What is your fix for the problem, implemented in this PR?

I added a man page for `bundle doctor`.

### Why did you choose this fix out of the possible options?

I chose this fix because the ["Writing docs for man pages" documentation](https://github.com/bundler/bundler/blob/master/doc/documentation/WRITING.md#what-goes-in-man-pages) says...

> Our goal is to have a man page for every command.

(cherry picked from commit b26714b1f6f6dbf2f7d5c4e5c09160e25ad4e034)

dilumnavanjana/bundler,mvz/bundler,dilumnavanjana/bundler,mvz/bundler,bundler/bundler,bundler/bundler