Auto merge of #8456 - ehuss:embedded-man, r=alexcrichton

Display embedded man pages for built-in commands.

This changes `cargo help COMMAND` to display the man page for the given command.  `cargo COMMAND --help` continues to show the basic clap output.

The man pages are embedded in the executable in a compressed format. There's also a copy of the man pages in text format for platforms that do not have the `man` executable (like Windows).

It is unfortunate to check in more pre-generated files. I hope in the future that the usage of asciidoc can be replaced with something else (possibly a custom markdown-based solution).

cc #6104