Improve documentation

[PerFrivik]

Improve documentation. Why can't there be message generation like in ROS. Lets say I want to know what nav_msgs/Odometry is, all I need to do is google the message type and I get this beautiful thing:

https://docs.ros.org/en/noetic/api/nav_msgs/html/msg/Odometry.html

I tried the same for gz::msgs::Actuators, but did not find anything and had to go far into the source code to find anything at all.

[mjcarroll]

For versions up to garden, the documentation is available in docs.gazebosim.org, for example: Odometry or Actuators

In harmonic forward, since the message generation pipeline has changed, we are missing some of the documentation on generated message types, which has been reported here: #399

Additionally, there is on-going work to improve the documentation site to address things like search engine optimization to easily surface this information.

I'm going to close this as a duplicate of #399 and use that to track this for now. Thank you for reporting.

[PerFrivik]

Alright, thank you!