Declare our public API

We should formally declare what's part of our public API, so that users are aware of what will be kept stable and what's subject to change without warning or tick-tock.

Take into account:

* Public headers and what it means if they're in a `details` folder
* CMake functions from `gz-cmake`
* CLI arguments
* Binary symbols
* Protobuf namespaces

A good reference is [REP-2004](https://www.ros.org/reps/rep-2004.html#public-api) and the quality declarations of existing packages, like [rclcpp](https://github.com/ros2/rclcpp/blob/master/rclcpp/QUALITY_DECLARATION.md#public-api-declaration-1iii).

We could place this documentation in [releases](https://github.com/ignitionrobotics/docs/blob/master/releases.md) together with versioning and deprecation strategies.