Auto merge of #6565 - SamWhited:add_srht_ci_config, r=alexcrichton
Auto merge of #6565 - SamWhited:add_srht_ci_config, r=alexcrichton

Add builds.sr.ht CI config to book

This adds a CI configuration for [builds.sr.ht](https://builds.sr.ht) to the book alongside the existing configurations for Travis and GitLab CI.
The config in question is broadly similar to those two, except that I also opted to build the docs as I commonly find markdown oddities this way from the warnings it spits out.
I picked Arch Linux as the base image because it's easy to install rustup there (since there's a package for it in the Arch repos), but it could have been Debian or FreeBSD or something else just as easily if you have strong opinions about manually installing rustup.

**EDIT:** None of the other build configs had it, so I didn't add publishing tagged commits to crates.io using its secrets mechanism to log cargo in. However, if that's something you think would be beneficial, I can add it.

nipunn1313/cargo,tshepang/cargo,rust-lang/cargo,eminence/cargo,tsurai/cargo,tsurai/cargo,vojtechkral/cargo,tsurai/cargo,rust-lang/cargo,eminence/cargo,nipunn1313/cargo,vojtechkral/cargo,vojtechkral/cargo,tshepang/cargo,eminence/cargo,tshepang/cargo,rust-lang/cargo,rustyhorde/cargo,eminence/cargo,rustyhorde/cargo,rust-lang/cargo,tshepang/cargo,nipunn1313/cargo,tsurai/cargo,vojtechkral/cargo,nipunn1313/cargo,rustyhorde/cargo,rustyhorde/cargo,tsurai/cargo