fix(scaffolder): render TechDocs link on Template List page for TechDocs annotations

Show “View TechDocs” link when template has backstage.io/techdocs-ref or backstage.io/techdocs-entity.
Append backstage.io/techdocs-entity-path when set.
Use buildTechDocsURL from @backstage/plugin-techdocs-react.
Add tests covering both annotations and path handling.
Update dependencies to include @backstage/plugin-techdocs-common and @backstage/plugin-techdocs-react.
Add sample TechDocs scaffolding to notifications-demo template.
Closes #29076.

Signed-off-by: David Fankhänel <david.fankhaenel@aeb.com>

plugins/scaffolder/package.json

@@ -72,6 +72,8 @@
     "@backstage/plugin-permission-react": "workspace:^",
     "@backstage/plugin-scaffolder-common": "workspace:^",
     "@backstage/plugin-scaffolder-react": "workspace:^",
+    "@backstage/plugin-techdocs-common": "workspace:^",
+    "@backstage/plugin-techdocs-react": "workspace:^",
     "@backstage/types": "workspace:^",
     "@codemirror/language": "^6.0.0",
     "@codemirror/legacy-modes": "^6.1.0",
@@ -109,6 +111,7 @@
     "@backstage/dev-utils": "workspace:^",
     "@backstage/plugin-catalog": "workspace:^",
     "@backstage/plugin-permission-common": "workspace:^",
+    "@backstage/plugin-techdocs": "workspace:^",
     "@backstage/test-utils": "workspace:^",
     "@testing-library/dom": "^10.0.0",
     "@testing-library/jest-dom": "^6.0.0",