Refactor dpnp includes and add extension example

[ndgrigorian]

This PR proposes a refactor of dpnp's includes and the introduction of an example extension, to be tested to make sure examples using dpnp and dpnp.tensor can be written.

The usm_ndarray_constants.h header and dpnp4pybind11.hpp are moved to dpnp/include, removing a relative include and aligning with previous dpctl behavior.

Also introduces dpnp-config.cmake, which enables find_package(Dpnp) out of the box, which is installed with dpnp, and --cmakedir command line option, which leads to the CMake config.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2941/index.html

[coveralls]

coverage: 78.236% (-0.01%) from 78.248% — add-libtensor-pybind11-example into master

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev0=py313h509198e_74 ran successfully.
Passed: 1357
Failed: 3
Skipped: 16