Documentation for library writers

- [x] #503
- [ ] Write a tutorial for integrating Treelite into a C++ project (using CMake).
- [ ] Write a tutorial for integrating Treelite into a Python package. In particular, library writers should be encouraged to produce `treelite.Model` object, rather than a custom object with a native Treelite handle.

Read issue #469 and the completed #503 item first, then scope the two unchecked tutorials: C++ integration using CMake and Python-package integration. Done means both tutorials are written and the Python guidance recommends producing a treelite.Model object rather than a custom object with a native Treelite handle.

Written by the indexing model from the issue text.