Declare our public API

[chapulina]

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 and the quality declarations of existing packages, like rclcpp.

We could place this documentation in releases together with versioning and deprecation strategies.