Add sphinx as doc target.

cmake/FindSphinx.cmake

@@ -0,0 +1,13 @@
+# Via https://devblogs.microsoft.com/cppblog/clear-functional-c-documentation-with-sphinx-breathe-doxygen-cmake/
+
+# Look for an executable called sphinx-build
+find_program(SPHINX_EXECUTABLE
+             NAMES sphinx-build
+             DOC "Path to sphinx-build executable")
+ 
+include(FindPackageHandleStandardArgs)
+ 
+# Handle standard arguments to find_package like REQUIRED and QUIET
+find_package_handle_standard_args(Sphinx
+                                  "Failed to find sphinx-build executable"
+                                  SPHINX_EXECUTABLE)