(532) Add documentation for Content Block Manager

docs/content_block_manager.md

@@ -0,0 +1,39 @@
+# Content Block Manager
+
+## What is it?
+
+The Content Block Manager is a "mini application" within Whitehall, packaged as a Rails Engine.
+
+## Where is it?
+
+All the code (including tests) for the engine is located at [`lib/engines/content_block_manager`](https://github.com/alphagov/whitehall/tree/main/lib/engines/content_block_manager). 
+
+## Why is is packaged this way?
+
+The rationale for packaging the code in this way can be found in [ADR-0004](https://github.com/alphagov/whitehall/blob/main/docs/adr/0004-content-object-store-added-with-a-rails-engine.md)
+
+## How to access the Content Block Manager
+
+The Content Block Manager is listed as an app within Signon, separate from Whitehall.
+
+The Content Block Manager link on signon integration was generated by running the below command in signon kubernetes 
+(refer to the [signon usage documentation for details](https://docs.publishing.service.gov.uk/repos/signon/usage.html)).
+
+```
+rake applications:create name="Content block manager" description="Create, edit and use modular content" \
+     home_uri="https://whitehall-admin.integration.publishing.service.gov.uk/content-block-manager" \
+     redirect_uri="https://whitehall-admin.integration.publishing.service.gov.uk/content-block-manager/auth/gds/callback"
+```
+
+This was inspired by the [Mainstream team's work on the Homepage app within Publisher](https://github.com/alphagov/publisher/blob/main/docs/homepage.md).
+As with the Homepage app, permissions are managed through the Whitehall app, rather than the "app" itself.
+
+## Why is this being added directly to the Signon landing page?
+
+The functionality itself will sit within Whitehall. However, we’ve made the decision to make the Content Block Manager
+behave as a seperate app, because we (like the Mainstream team) believe that users should simply be able to select what 
+action they want to perform, rather than having to understand what app they need to use to accomplish a task. 
+Additionally, the navigation within Whitehall is already very crowded with a lot of functionality buried underneath the 
+"More" menu, and we don't want to add to this noise
+
+We have already done some light touch testing with users, and we believe this approach is widely understood and agreed.