techdocs: skeleton of reader (#1438)

* chore: replaced default components

* feat(techdocs): added skeleton of reader

* fix: added types

* fix: split out into separate file

* fix: add shadowDom hook test

* fix: added @backstage/test-utils as dep

* fix: formatting python files

plugins/techdocs/package.json

@@ -22,6 +22,7 @@
   },
   "dependencies": {
     "@backstage/core": "^0.1.1-alpha.12",
+    "@backstage/test-utils": "^0.1.1-alpha.12",
     "@backstage/theme": "^0.1.1-alpha.12",
     "@material-ui/core": "^4.9.1",
     "@material-ui/icons": "^4.9.1",