Add link to NVStore.md

Add storage config page to related content for good link juice and a better user experience.

Author: Amanda Butler

docs/reference/api/storage/NVStore.md

@@ -15,3 +15,7 @@ NVStore uses two Flash areas, active and nonactive. Each area must consist of at
 ### NVStore example
 
 [![View code](https://www.mbed.com/embed/?url=https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-nvstore/)](https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-nvstore/file/0a3e19b12855/main.cpp)
+
+### Related content
+
+- [Storage configuration](configuration-storage.html).