Add related content to FileSystem.md

Add related content for consistent UX.

Author: Amanda Butler

docs/reference/api/storage/FileSystem.md

@@ -9,3 +9,7 @@ The file system API provides a common interface for implementing a file system o
 ### File system example
 
 [![View code](https://www.mbed.com/embed/?url=https://github.com/armmbed/mbed-os-example-fat-filesystem)](https://github.com/ARMmbed/mbed-os-example-fat-filesystem/blob/master/main.cpp)
+
+### Related content
+
+- [Storage configuration](configuration-storage.html).