Add new section for Neutron Internals

This section contains some notes for each layer of Neutron that
I have done deep dives on
[WIP] Cleanup Security Group API Extension tour

Convert from confluence wiki syntax to markdown

Change-Id: Iea4932baa637640a486b22dec8fd96356827318e
Make the Security Group API extension an example

Change-Id: Id70734536a910e4be74a88a593c18f4d994654f5
Shuffle some content around

Change-Id: I4f2ebce3a8010cdc6a076a9e91ab82a703556a72
Clean up Security Group API section some more

Change-Id: I7c43ad59a0be8e2bad4e8125bf67828822a8de5d
Make automodule a sub-section

Change-Id: I149a2a126736d51fc1e09a06b8c0488f8adbfe16