Reorganize src, tests, and benchmarks

[harrism]

Description

Fixes #663.
Also fixes #1105

Organizes all source, test, and benchmark files into directories matching the names of the associated headers that define the APIs they implement/test/benchmark.

Also fixes up documentation so it is consistent with this.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[trxcllnt]

cpp/tests/distance/polygon_distance_test.cpp:21:10: fatal error: cuspatial/distance/polygon_distance.hpp: No such file or directory
   21 | #include <cuspatial/distance/polygon_distance.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

[harrism]

/merge

[isVoid]

Looks like cython file for polygon_distance require updating.

$SRC_DIR/python/cuspatial/_skbuild/linux-x86_64-3.9/cmake-build/cuspatial/_lib/distance.cxx:792:10: fatal error: cuspatial/distance/polygon_distance.hpp: No such file or directory
  792 | #include "cuspatial/distance/polygon_distance.hpp"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.