techdocs: add missing alpha entity content route ref

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/fifty-turtles-count.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-techdocs': patch
+---
+
+Add missing route ref to the `/alpha` entity content extension.

plugins/techdocs/src/alpha.tsx

@@ -170,6 +170,7 @@ const techDocsEntityContent = EntityContentBlueprint.makeWithOverrides({
       {
         defaultPath: 'docs',
         defaultTitle: 'TechDocs',
+        routeRef: convertLegacyRouteRef(rootCatalogDocsRouteRef),
         loader: () =>
           import('./Router').then(({ EmbeddedDocsRouter }) =>
             compatWrapper(