Document gazebo client connection with gazebo server

# Overview
The gazebo server is running in a docker container in Headless mode, however, if you have installed gazebo client on your host machine you can connect to the server.

# Benefits

This will allow people to view and manipulate the gazebo environment, for example to:
- observe the drone while it's flying
- manipulate the simulation environment by adding or moving objects and other features that the client allows for.

# What you should do

Document the command we can use to connect to the server.

# Tasks
Explain how to connect a gazebo client to the server in:

- [x] ~README~ (not really needed)
- [x] book - Add a section in the book

# Bugs

- [ ] ign-transport not working
- [ ] Models don't load - because they need to be loaded from the host machine