Add doc(html_root_url) so links from external crates to time work

[SergioBenitez]

See https://doc.rust-lang.org/rustdoc/the-doc-attribute.html#html_root_url.

[jhpratt]

While this has been recommended by the API guidelines for quite a while, this recommendation is pending removal; it has already completed FCP and just needs merging. So unless there is something everyone has missed, including during the announced FCP, I'm inclined to close this.

Reference

[SergioBenitez]

I've expressed my thoughts in the issue. For concreteness, links to time are currently broken in the development docs because this attribute is missing from time.

[jhpratt]

Ok, I'll wait for some further discussion to take place there. I will refrain from duplicating discussion as well.

[SergioBenitez]

It looks like the new-ish -Zrustdoc-map lets us fill in the URL on our side. As long as the docs are at docs.rs, this'll do. Thanks!