List internal topics in overview

[doc only]

docs/dev/core/internal/index.md

@@ -2,8 +2,11 @@
 
 This chapter describes aspects of Glean that are internal implementation details.
 
-This includes
+This includes:
 
-* [Reserved ping names](reserved-ping-names.md).
-* [Clearing metrics when disabling/enabling Glean](clearing.md).
-* [Payload format](payload.md).
+* [Reserved ping names](reserved-ping-names.md)
+* [Clearing metrics when disabling/enabling Glean](clearing.md)
+* [Payload format](payload.md)
+* [Directory structure](directory-structure.md)
+* [Debug Pings](debug-pings.md)
+* [Implementations](implementations.md)