Change redirect logic

When Docusaurus detects a section index page - in which the slug matches
the second-to-last URL path segment - it produces a route for the page
with only the second-to-last path segment, omitting the slug. When
migrating to the Docusaurus site, we added redirects from paths with the
format `/slug/slug/` to `/slug/`.

This change edits these redirects progammatically to reverse the
source/destination mappings. If a redirect only maps a Docusaurus-style
slug to a pre-migration index page slug, the docs engine rewrites it to
map the pre-migration slug to a Docusaurus-style one.

For redirect destinations that use pre-migration slugs, this change
adjusts them to be Docusaurus-style slugs (this is the current redirect
rewriting behavior).

server/redirects.ts

@@ -14,18 +14,42 @@ export const getRedirects = () => {
 
   return result.map((redirect) => {
     // If a page is an index page for a section, it has the same name as the
-    // containing subdirectory. If a redirect destination is a menu page,
-    // rewrite it to point to the containing URL path so Docusaurus recognizes
-    // it as a valid redirect.
-    const pathSegs = redirect.destination
+    // containing subdirectory. Handle this logic to accommodate redirects
+    // from the previous docs site.
+    const sourceSegs = redirect.source
       .replaceAll(new RegExp("/$", "g"), "")
       .split("/");
-    if (
-      pathSegs.length >= 2 &&
-      pathSegs[pathSegs.length - 1] == pathSegs[pathSegs.length - 2]
-    ) {
-      redirect.destination =
-        pathSegs.slice(0, pathSegs.length - 2).join("/") + "/";
+
+    const destSegs = redirect.destination
+      .replaceAll(new RegExp("/$", "g"), "")
+      .split("/");
+
+    // The destination is an index page, since the slug matches the containing
+    // path segment.
+    const destIndex =
+      destSegs.length >= 2 &&
+      destSegs[destSegs.length - 1] == destSegs[destSegs.length - 2];
+
+    if (!destIndex) {
+      return {
+        from: redirect.source,
+        to: redirect.destination,
+      };
+    }
+
+    const docusaurusDest =
+      destSegs.slice(0, -1).join("/") + "/";
+
+    // This redirect maps a Docusaurus-style index page to a previous-site index
+    // page, so reverse the mapping.
+    if (redirect.source == docusaurusDest) {
+      const oldSource = redirect.source;
+      redirect.source = redirect.destination;
+      redirect.destination = oldSource;
+      // This redirect has an old-style index page path as a destination, so
+      // change it to a Docusaurus-style one.
+    } else {
+      redirect.destination = docusaurusDest;
     }
 
     return {