Set html_root_url attribute

[tspiteri]

https://rust-lang.github.io/api-guidelines/documentation.html#crate-sets-html_root_url-attribute-c-html-root

[Lokathor]

I asked @jyn514 and they said that this is apparently not a thing people should use in the common case, but the PR to update the docs about it has been stalled: rust-lang/api-guidelines#230

[tspiteri]

Ok. In my case apart from the docs.rs documentation (which uses extern-html-root-url) I keep a copy of the docs for the latest git default branch. The links were not working there, hence this PR. In the meantime I had updated my doc update script to use extern-html-root-url, so that solves the issue for me. So I think this PR can be closed.

[jyn514]

In the meantime I had updated my doc update script to use extern-html-root-url, so that solves the issue for me.

On nightly you can use -Z rustdoc-map which will do this automatically.