Fix broken crates.html links to actual API doc locations

Item-level links (structs, traits, fns, macros) were pointing to
api/rustmax/{crate}/ which only contains index.html re-export pages.
Fixed linksubs.json5 to point to actual item locations under api/{crate}/,
including cross-crate remaps (crossbeam->crossbeam_channel, serde->serde_core,
clap->clap_builder, etc.) and std->core for core traits. Fixed
substitute_versions() to use api/ instead of api/rustmax/ prefix.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: brson

crates/rustmax-prebuild/src/main.rs

@@ -487,7 +487,7 @@ fn substitute_versions(md: &str, crates: &[CrateInfo]) -> String {
             let link_tail = link_tail.as_str().trim();
             if find_crate(crates, crate_name).is_some() {
                 buf.push_str(&format!(
-                    "[{link_name}]: api/rustmax/{link_tail}"
+                    "[{link_name}]: api/{link_tail}"
                 ));
             } else {
                 buf.push_str(line);

crates/rustmax/doc-src/crate-bitflags.md

@@ -3,7 +3,7 @@ Types in which the bits are individually addressable.
 - Crate [`::bitflags`].
 - [docs.rs](https://docs.rs/bitflags)
 - [crates.io](https://crates.io/crates/bitflags)
-- [GitHub](todo)
+- [GitHub](https://github.com/bitflags/bitflags)
 
 ---