docs error more explanatory

Signed-off-by: Brian Fletcher <brian@roadie.io>

.changeset/flat-peaches-act.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-techdocs-backend': patch
+---
+
+Techdocs backend explains a bit more about about what might have caused the docs not being found when building techdocs.

plugins/techdocs-backend/src/service/DocsSynchronizer.ts

@@ -165,7 +165,7 @@ export class DocsSynchronizer {
       );
       error(
         new NotFoundError(
-          'Sorry! It took too long for the generated docs to show up in storage. Check back later.',
+          'Sorry! It took too long for the generated docs to show up in storage. Are you sure the docs project is generating an `index.html` file? Otherwise, check back later.',
         ),
       );
       return;