Add screenshots of simulation demos

[chapulina]

The Ignition home page could have some images to show prospective users what the simulator is capable of. Both the home page and the features page could use screenshots showcasing different robots and environments being simulated.

Related, we don't show the version logos anywhere on the site! Maybe each collection could have the logo on its entry page, here's Citadel's page.

[chapulina]

Some possible things to screenshot by Edifice's release:

• An indoor environment using the latest lightmaps / environment maps, like in Add lightmap demo gz-sim#471
• Underground robots (quadcopter, wheeled, tracked, legged) and caves from SubT
• A humanoid robot, like in Add joint position controller GUI, also enable tests for GUI plugins gz-sim#534 - it would be nice to see it walking
• An industrial arm, like in Add JointTrajectoryController system plugin gz-sim#473
• Mecanum wheels, coming from Demo for edifice: mecanum wheels gz-sim#579
• A submarine in a world with a heightmap on the ocean floor

[chapulina]

We now have these pages:

• https://ignitionrobotics.org/showcase
• https://ignitionrobotics.org/media

And the home page also has some nice images:

https://ignitionrobotics.org/home