Currently docs can only be generated if morpheus_llm is on

Added a message in the main CMakelists.txt t check for this and fail the
build early. Sample output -
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Runing CMake configure...
+ cmake -S . -B build -GNinja -DCMAKE_MESSAGE_CONTEXT_SHOW=ON
  -DMORPHEUS_USE_CLANG_TIDY=OFF -DMORPHEUS_PYTHON_INPLACE_BUILD=ON
-DMORPHEUS_PYTHON_PERFORM_INSTALL=ON -DMORPHEUS_USE_CCACHE=ON
-DMORPHEUS_USE_CONDA=ON -DMORPHEUS_SUPPORT_DOCA=OFF
-DMORPHEUS_BUILD_MORPHEUS_LLM=ON
-DCMAKE_AR=/home/ubuntu/miniforge3/envs/comp-llm/bin/x86_64-conda-linux-gnu-ar
-DCMAKE_CXX_COMPILER_AR=/home/ubuntu/miniforge3/envs/comp-llm/bin/x86_64-conda-linux-gnu-gcc-ar
-DCMAKE_C_COMPILER_AR=/home/ubuntu/miniforge3/envs/comp-llm/bin/x86_64-conda-linux-gnu-gcc-ar
-DCMAKE_RANLIB=/home/ubuntu/miniforge3/envs/comp-llm/bin/x86_64-conda-linux-gnu-ranlib
-DCMAKE_CXX_COMPILER_RANLIB=/home/ubuntu/miniforge3/envs/comp-llm/bin/x86_64-conda-linux-gnu-gcc-ranlib
-DCMAKE_C_COMPILER_RANLIB=/home/ubuntu/miniforge3/envs/comp-llm/bin/x86_64-conda-linux-gnu-gcc-ranlib
-DCMAKE_LINKER=/home/ubuntu/miniforge3/envs/comp-llm/bin/x86_64-conda-linux-gnu-ld
-DCMAKE_STRIP=/home/ubuntu/miniforge3/envs/comp-llm/bin/x86_64-conda-linux-gnu-strip
-DMORPHEUS_BUILD_DOCS=ON -DMORPHEUS_BUILD_MORPHEUS_LLM=OFF
CMake Error at CMakeLists.txt:51 (message):
  MORPHEUS_BUILD_MORPHEUS_LLM must be ON if MORPHEUS_BUILD_DOCS is ON
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

-- Configuring incomplete, errors occurred!

Signed-off-by: Anuradha Karuppiah <anuradhak@nvidia.com>

CMakeLists.txt

@@ -47,6 +47,10 @@ if(MORPHEUS_USE_IWYU AND MORPHEUS_USE_CCACHE)
   message(FATAL_ERROR "MORPHEUS_USE_IWYU and MORPHEUS_USE_CCACHE cannot be set simultaneously")
 endif()
 
+if (MORPHEUS_BUILD_DOCS AND NOT MORPHEUS_BUILD_MORPHEUS_LLM)
+  message(FATAL_ERROR "MORPHEUS_BUILD_MORPHEUS_LLM must be ON if MORPHEUS_BUILD_DOCS is ON")
+endif()
+
 # MRC CMake path and module extensions
 set(MORPHEUS_CMAKE_MODULE_PATH_EXTENSIONS
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"