Display actual rust version used on docs.rs about page

[dekellum]

Assuming that for stability reasons, docs.rs is not going to be updated every night to the very latest nightly, it would be helpful if there was a known/easy place to see what version docs.rs was actually using. This would allow testing cargo doc on the same version, manually or via CI, to confirm it will build on docs.rs before a release, instead of being surprised by a docs.rs build failure after release.

As per this #233 comment by @QuietMisdreavus, on 2018-10-25:

Now that #243 is merged, docs.rs is using 1.31.0-nightly (78ff609d7 2018-10-19) as its rustc/rustdoc version.

Consider changing the following line on https://docs.rs/about:

Docs.rs automatically builds crates' documentation released on crates.io using the nightly release of the Rust compiler.

To something like:

Docs.rs automatically builds crates' documentation released on crates.io, currently using rust 1.31.0-nightly (78ff609d7 2018-10-19). If your project needs a later version, check existing issues and create a new issue if appropriate.

In addition, there could be some CI or other automation utility for something like https://docs.rs/about/version returning a text/plain or application/json version string.