create stack-overflow backend plugin and move collator there

Signed-off-by: Emma Indal <emma.indahl@gmail.com>

plugins/stack-overflow-backend/README.md

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+# Stack Overflow
+
+A plugin that provides stack overflow specific functionality that can be used in different ways (e.g. for search) to compose your Backstage App.
+
+## Getting started
+
+Before we begin, make sure:
+
+- You have created your own standalone Backstage app using @backstage/create-app and not using a fork of the backstage repository. If you haven't setup Backstage already, start [here](https://backstage.io/docs/getting-started/).
+
+To use any of the functionality this plugin provides, you need to start by configuring your App with the following config:
+
+```yaml
+stackoverflow:
+  baseUrl: https://api.stackexchange.com/2.2 # alternative: your internal stack overflow instance
+```
+
+## Areas of Responsibility
+
+This stack overflow backend plugin is primarily responsible for the following:
+
+- Provides a `StackOverflowQuestionsCollatorFactory`, which can be used in the search backend to index stack overflow questions to your Backstage Search.
+
+### Index Stack Overflow Questions to search
+
+Before you are able to start index stack overflow questions to search, you need to go through the [search getting started guide](https://backstage.io/docs/features/search/getting-started).
+
+When you have your `packages/backend/src/plugins/search.ts` file ready to make modifications, add the following code snippet to add the `StackOverflowQuestionsCollatorFactory`. Note that you can modify the `requestParams`.
+
+> Note: if your baseUrl is set to the external stack overflow api `https://api.stackexchange.com/2.2`, you can find optional and required params under the official API documentation under [`Usage of /questions GET`](https://api.stackexchange.com/docs/questions)
+
+```ts
+indexBuilder.addCollator({
+  defaultRefreshIntervalSeconds: 600,
+  factory: StackOverflowQuestionsCollatorFactory.fromConfig(config, {
+    requestParams: {
+      tagged: ['backstage'],
+      site: 'stackoverflow',
+      pagesize: 100,
+    },
+  }),
+});
+```