Fixed s3 path join to work under any os

Signed-off-by: Andre Wanlin <awanlin@rapidrtc.com>

.changeset/tender-bugs-sort.md

@@ -0,0 +1,5 @@
+---
+'@backstage/techdocs-common': patch
+---
+
+Updated to properly join URL segments under any OS

packages/techdocs-common/src/stages/publish/awsS3.ts

@@ -357,7 +357,7 @@ export class AwsS3Publish implements PublisherBase {
         : lowerCaseEntityTripletInStoragePath(decodedUriNoRoot);
 
       // Re-prepend the root path to the relative file path
-      const filePath = path.join(this.bucketRootPath, filePathNoRoot);
+      const filePath = path.posix.join(this.bucketRootPath, filePathNoRoot);
 
       // Files with different extensions (CSS, HTML) need to be served with different headers
       const fileExtension = path.extname(filePath);