Fix up links to markdown files from within other markdown files in the Rust docs

Our language reference .md file contains a refernce to
builtin_elements.md. Since rustdoc doesn't further process the embedded
markdown file, the .md link remains as-is and is broken. But we can fix
it up using a small snippet of JavaScript.

Fixes #87

api/sixtyfps-rs/sixtyfps-docs-integration.html

@@ -40,5 +40,19 @@
     }
     run();
 
+    // Included markdown files may have links to other markdown files, which may not have been
+    // resolved by rustdoc. This helper locates such links and resolves them, assuming that each
+    // .md file gets its own sub-directory with an index.html.
+    function fix_markdown_links() {
+        for (let anchor of document.querySelectorAll('a[href$=".md"]')) {
+            let url = new URL(anchor.href);
+            let dir_separator = Math.max(url.pathname.lastIndexOf("/"), 0);
+            let base_name = url.pathname.slice(dir_separator + 1, -3);
+            let base_path = url.pathname.slice(0, dir_separator);
+            url.pathname = base_path + "/../" + base_name + "/index.html";
+            anchor.setAttribute("href", url.pathname);
+        }
+    }
+    fix_markdown_links()
 
 </script>