Draft: Add sphinx documentation

The mechanism for building docs was copied from EveryBeam; it parses both Doxygen documentation and python documentation into a sphinx page.

cmake/FindSphinx.cmake

+# CMake find_package() Module for Sphinx documentation generator
+# http://sphinx-doc.org/
+#
+# Example usage:
+#
+# find_package(Sphinx)
+#
+# If successful the following variables will be defined SPHINX_FOUND
+# SPHINX_EXECUTABLE
+
+find_program(
+  SPHINX_EXECUTABLE
+  NAMES sphinx-build sphinx-build2
+  DOC "Path to sphinx-build executable")
+
+# Handle REQUIRED and QUIET arguments this will also set SPHINX_FOUND to true if
+# SPHINX_EXECUTABLE exists
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(
+  Sphinx "Failed to locate sphinx-build executable" SPHINX_EXECUTABLE)
+
+# Provide options for controlling different types of output
+option(SPHINX_OUTPUT_HTML "Output standalone HTML files" ON)
+option(SPHINX_OUTPUT_MAN "Output man pages" ON)
+
+option(SPHINX_WARNINGS_AS_ERRORS
+       "When building documentation treat warnings as errors" ON)