Rename Ignition to Gazebo

[srmainwaring]

Rename the folder containing example models for Gazebo following this announcement: A new era for Gazebo.

• Closes #92
• Part of Gazebo with SITL Documentation Is Outdated #1995.

Checks

• [x] Check Truck instructions.
• [x] Check Quadruped instructions.
• [x] Check RoverPlaypen instructions.
• [x] Check SkyCatTVBS instructions.
• [x] Check SkywalkerX8 instructions.
• [x] Check WildThumper instructions.

[srmainwaring]

@tridge I think this is low risk to merge without a review - so I'll go ahead as I'd like to cross reference the examples in other docs using the renamed locations.