[docs][rust] Update rust API docs link

The previous link was to an arbitrary (although important) crate.
Instead, point it at rustdoc_index, which is actually intended to serve
as an index page.

Bug: 441377312
Change-Id: Ibf5b4044e9ff54840c41d1d88bcad37ad703671a
Reviewed-on: https://fuchsia-review.googlesource.com/c/api-docs-resources/+/1510136
Reviewed-by: Megan Batty <mgnb@google.com>