Implement DefaultTechDocsCollator

* Implements a collator for tech docs.
   * Retrieves mkdocs created search index for entities that have documentation configured
* Registers collator to expose tech docs content to be searchable
* Adds pagination to example search
* Modifies example search to contain tech docs
   * Displays docs results with link to docs and the entity name as title.
* Creates a reusable type filter to be located in the search package.
* Add tests for type filter

Signed-off-by: Jussi Hallila <jussi@hallila.com>

plugins/techdocs-backend/package.json

@@ -35,6 +35,7 @@
     "@backstage/catalog-model": "^0.9.0",
     "@backstage/config": "^0.1.5",
     "@backstage/errors": "^0.1.1",
+    "@backstage/search-common": "^0.1.2",
     "@backstage/techdocs-common": "^0.6.8",
     "@types/express": "^4.17.6",
     "cross-fetch": "^3.0.6",
@@ -43,10 +44,13 @@
     "express-promise-router": "^4.1.0",
     "fs-extra": "9.1.0",
     "knex": "^0.95.1",
+    "lodash": "^4.17.21",
+    "p-limit": "^3.1.0",
     "winston": "^3.2.1"
   },
   "devDependencies": {
     "@backstage/cli": "^0.7.4",
+    "@backstage/test-utils": "^0.1.14",
     "@types/dockerode": "^3.2.1",
     "msw": "^0.29.0",
     "supertest": "^6.1.3"