Add mdman for generating man pages.

[ehuss]

This introduces a new utility called mdman that converts a markdown-formatted document to a man page. This replaces asciidoctor, with the intent to make it easier to contribute, easier to have consistent formatting across platforms, and easier to generate plain-text documents for use on Windows (for #8456). This also includes a number of formatting fixes.

There is some documentation in the mdman/doc directory explaining how to use it, and the docs in src/doc/README.md have been updated (this explains the structure of the files). The Makefile has been replaced with a simple shell script.

CI has been updated to verify the checked-in docs are up-to-date. Perhaps in the future, these can be generated automatically (perhaps by build.rs?), but since that requires a bit of build system work (like upstream rust), this is deferred till later.

[rust-highfive]

r? @Eh2406

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[alexcrichton]

Nice! I didn't review mdman too too closely but everything looks pretty reasonable at a first glance, and all the Cargo integration bits look good to me. I'd be happy to basically land this whenever myself, unless you've got something else you'd like to take care of!

[ehuss]

Thanks for taking a look! And sorry it's such a big diff. I did split it up into multiple commits to try to organize things a little. If any issues come up, I'll try to take care of them. I don't have anything else here, so:
@bors r=alexcrichton

[bors]

📌 Commit 566706e has been approved by alexcrichton

[bors]

⌛ Testing commit 566706e with merge e928120...

[bors]

☀️ Test successful - checks-actions
Approved by: alexcrichton
Pushing e928120 to master...