docs: add external reference material

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

docs/index.md

@@ -12,3 +12,9 @@ development/options.md
 development/prf.md
 development/qemu.md
 ```
+
+```{toctree}
+:hidden:
+:caption: 📖 Reference 
+reference/external.md
+```

docs/reference/external.md

@@ -0,0 +1,7 @@
+# External
+
+Here you will find a list of relevant external resources related to PebbleOS.
+
+- [Pebble OS capabilities](https://docs.google.com/presentation/d/1wfyBRwbrv5YtSnvNRnEPz5tRx9y7VGcFsuHbi1X-D7I/edit?usp=sharing)
+- [Firmware Development (For Non-Firmware Engineers)](https://docs.google.com/presentation/d/1M--yoEJBO-uckvY5CTFfHT4srw6RCj9RTGT57RcogX8/edit?usp=sharing)
+- [COREDUMP #003: Pebble's Code is Free: 3 Former Pebble Engineers Discuss Why It's Important (Pt. 1/2)](https://www.youtube.com/watch?v=dk5wsNN8abo)