Define a stable C++ API

[dbaston]

The stability of the GEOS C++ API is described here: https://libgeos.org/usage/cpp_api/

Essentially, you're probably OK to write a program that uses core methods of Geometry, WKTReader, etc. and build it against different versions of GEOS. If you write a program that relies on a method of OverlayGraph, it might break between versions.

While this description is accurate, it would be helpful if we could flag the methods/classes that can be considered "stable" via a doxygen tag or some other mechanism.

Ideas welcome.