Fix doc links

[dhardy]

Closes #1077:

• add a redirect at https://rust-random.github.io/rand/ (have to test this live)
• use docs.rs for documentation links, since people usually want doc corresponding to the published version not the master branch
• use the book as the "homepage" since linking back to the crate pages is not useful and we already have a repo link

[dhardy]

PS once approved, I'll push equivalent changes into the rngs repo.

[newpavlov]

Looks good!

[dhardy]

The redirect doesn't work: URL is still 404. Did I get the path wrong?

[newpavlov]

It redirects me without problem. Likely it got updated a bit later.