Fixed bug with the index page's relative links.

[HU90m]

If there is no index page in the source directory, mdbook uses the first chapter as the index page. When doing so, mdbook doesn't render the page any differently. This causes relative links on the page to break, if the first chapter is within a sub-directory and there is not index.md (or README.md) in the root.

To fix this, a redirect to the first chapter is used instead of a copy of the page.

See bug report for more information: https://github.com/rust-lang/mdBook/issues/2060

[KFearsoff]

@rustbot label -S-waiting-on-review

[nbdd0121]

Not sure why the tag is removed...

@rustbot label +S-waiting-on-review