Separate getting started docs into Python and C++ (#364)

[mihaic]

Separate getting started docs into Python and C++ (#364)

There are a few additional related changes:

• Remove Python dependency for C++ example by using test data.
• Move C++ Vamana test from Python to C++ tests.
• Simplify main README and add READMEs for examples.

Also pin NumPy until we ensure 2.0 compatibility.