fix(sidebars): use startsWith to extract basePath instead of split (#1368)

String.split(doc) matched doc as a substring, so outputDir paths like
"docs/test/docspace" with docPath "docs" would split on the second
occurrence of "doc" inside "docspace", producing double slashes or
missing path segments in generated sidebar IDs.

Replacing with startsWith + substring ensures only the exact path prefix
is stripped at a path boundary.

Closes #1305

Author: sserrata

packages/docusaurus-plugin-openapi-docs/src/sidebars/index.ts

@@ -131,8 +131,8 @@ function groupByTags(
     output: string,
     doc: string | undefined
   ): string => {
-    if (doc && output.includes(doc)) {
-      return output.split(doc)[1]?.replace(/^\/+/g, "") ?? "";
+    if (doc && output.startsWith(doc + "/")) {
+      return output.substring((doc + "/").length);
     }
     const slashIndex = output.indexOf("/", 1);
     return slashIndex === -1