[docs] Use FetchContent to grab CMake modules (#61)

CMakeLists.txt

@@ -1,8 +1,15 @@
 cmake_minimum_required(VERSION 3.15)
 project(SphinxLLMsTxt VERSION 1.0.0 LANGUAGES NONE)
 
-# Add CMake module path
-list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
+# Fetch Sphinx CMake modules
+include(FetchContent)
+FetchContent_Declare(
+  sphinx_cmake_modules
+  GIT_REPOSITORY https://github.com/jdillard/sphinx-cmake-modules.git
+  GIT_TAG        main
+)
+FetchContent_MakeAvailable(sphinx_cmake_modules)
+list(APPEND CMAKE_MODULE_PATH "${sphinx_cmake_modules_SOURCE_DIR}")
 
 # Add documentation
 add_subdirectory(docs)