Use relative links instead of intra-doc links

Previously, `BTreeMap` tried to link to `crate::collections`, intending
for the link to go to `std/collections/index.html`. But `BTreeMap` is
defined in `alloc`, so after the fix in the previous commit, the links
instead went to `alloc/collections/index.html`, which has almost no
information.

This changes it to link to `index.html`, which only works when viewing
from `std::collections::BTreeMap`, the most common place to visit the
docs. Fixing it to work from anywhere would require the docs for
`std::collections` to be duplicated in `alloc::collections`, which in
turn would require HashMap to be `alloc` for intra-doc links to work
(#74481).

library/alloc/src/collections/btree/map.rs

@@ -811,7 +811,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
     /// types that can be `==` without being identical. See the [module-level
     /// documentation] for more.
     ///
-    /// [module-level documentation]: crate::collections#insert-and-complex-keys
+    /// [module-level documentation]: index.html#insert-and-complex-keys
     ///
     /// # Examples
     ///