Re-Structure cmake solution to be closer to the scons solution.

This is just a single step, re-arranging the code without actually changing its functionality.

new docs/cmake.md
moved the block of comments from the start of the CMakeLists.txt into the cmake.md file and converted content to markdown.

new cmake/godotcpp.cmake
Moved all exposed options into a new function godotcpp_options()
Moved configuration and generation code into godotcpp_generate()

To get all the options into the godotcpp_options() I changed the logic of GODOT_USE_HOT_RELOAD which I believe is a closer match to scons, that if the options is not set, and the build type is not release, then it defaults to ON.

CMakeLists.txt

@@ -5,7 +5,7 @@ project(godot-cpp LANGUAGES CXX)
 list( APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake/" )
 include( godotcpp )
 
-# I know this doesnt look like a typical CMakeLists.txt, but as we are
+# I know this doesn't look like a typical CMakeLists.txt, but as we are
 # attempting mostly feature parity with SCons, and easy maintenance, the closer
 # the two build systems look the easier they will be to keep in lockstep.