techdocs-backend: Remove individual preparers and generators from app backend, to use factory methods

When a new Backstage app is setup, the techdocs backend file contains all the preparers (dir, github, gitlab, etc.) and same for generators. While this providers added customizibility, it comes with a cost of complexity in setting up the app backend. New preparers' updates would need users to update their app's techdocs backend file.

Scaffolder backend has already moved to this proposed pattern.

packages/backend/src/plugins/techdocs.ts

@@ -15,12 +15,8 @@
  */
 import {
   createRouter,
-  DirectoryPreparer,
   Preparers,
   Generators,
-  TechdocsGenerator,
-  CommonGitPreparer,
-  UrlPreparer,
   Publisher,
 } from '@backstage/plugin-techdocs-backend';
 import { PluginEnvironment } from '../types';
@@ -33,30 +29,25 @@ export default async function createPlugin({
   reader,
 }: PluginEnvironment) {
   // Preparers are responsible for fetching source files for documentation.
-  const preparers = new Preparers();
-
-  const directoryPreparer = new DirectoryPreparer(logger);
-  preparers.register('dir', directoryPreparer);
-
-  const commonGitPreparer = new CommonGitPreparer(logger);
-  preparers.register('github', commonGitPreparer);
-  preparers.register('gitlab', commonGitPreparer);
-  preparers.register('azure/api', commonGitPreparer);
-
-  const urlPreparer = new UrlPreparer(reader, logger);
-  preparers.register('url', urlPreparer);
+  const preparers = await Preparers.fromConfig(config, {
+    logger,
+    reader,
+  });
 
   // Generators are used for generating documentation sites.
-  const generators = new Generators();
-  const techdocsGenerator = new TechdocsGenerator(logger, config);
-  generators.register('techdocs', techdocsGenerator);
+  const generators = await Generators.fromConfig(config, {
+    logger,
+  });
 
-  // Publishers are used for
+  // Publisher is used for
   // 1. Publishing generated files to storage
   // 2. Fetching files from storage and passing them to TechDocs frontend.
-  const publisher = Publisher.fromConfig(config, logger, discovery);
+  const publisher = await Publisher.fromConfig(config, {
+    logger,
+    discovery,
+  });
 
-  // Docker client used by the generators.
+  // Docker client (conditionally) used by the generators, based on techdocs.generators config.
   const dockerClient = new Docker();
 
   return await createRouter({