Resolve #259 - Add extensions to getting started guide

Author: craigfowler

CSF.Screenplay.Docs/docs/index.md

@@ -1,6 +1,8 @@
 # Get started with Screenplay
 
-To get started with Screenplay, the first step is to decide how you'd like to use it:
+## 1. Install Screenplay
+
+The first step is to decide how you'd like to use it:
 
 * [As a testing tool, with NUnit 3]
 * [As a testing tool, with Reqnroll]
@@ -11,3 +13,15 @@ To get started with Screenplay, the first step is to decide how you'd like to us
 [As a testing tool, with Reqnroll]: gettingStarted/reqnroll/index.md
 [the retired SpecFlow]: https://reqnroll.net/news/2025/01/specflow-end-of-life-has-been-announced/
 [As a process automation library]: gettingStarted/nonTesting/index.md
+
+## 2. Pick extensions
+
+To get the most from Screenplay you should install one or more [extensions].
+These give Screenplay the capability to interact with and control other systems.
+
+Here is [a list of the extensions which are authored alongside Screenplay's core].
+If you'd like, you are also encouraged to [write your own extension], adding new capabilities to Screenplay.
+
+[extensions]: ../glossary/Extension.md
+[a list of the extensions which are authored alongside Screenplay's core]: extensions/index.md
+[write your own extension]: extendingScreenplay/index.md