Adding Doxygen and CMake changes to support it

docs/CMakeLists.txt

@@ -0,0 +1,32 @@
+find_package(Doxygen)
+if (NOT DOXYGEN_FOUND)
+    message(FATAL_ERROR "Doxygen could not be sourced.")
+    return()
+endif()
+
+include(FetchContent)
+FetchContent_Declare(
+    doxygen-awesome-css
+    URL https://github.com/jothepro/doxygen-awesome-css/archive/refs/heads/main.zip
+)
+FetchContent_MakeAvailable(doxygen-awesome-css)
+FetchContent_GetProperties(doxygen-awesome-css SOURCE_DIR AWESOME_CSS_DIR)
+
+set(DOXYGEN_GENERATE_HTML YES)
+set(DOXYGEN_OUTPUT_DIRECTORY ${PROJECT_SOURCE_DIR}/docs)
+set(DOXYGEN_HTML_EXTRA_STYLESHEET ${AWESOME_CSS_DIR}/doxygen-awesome.css)
+
+if(ENVNB_SOURCE_FOUND)
+    message("Using file timestamps...")
+    doxygen_add_docs(NBEngineDocumentation
+        ${NB_CORE_SOURCE} ALL
+        USE_STAMP_FILE
+        COMMENT "Building documentation"
+    )
+else()
+    message("Processing all files...")
+    doxygen_add_docs(NBEngineDocumentation
+        ${PROJECT_SOURCE_DIR}/engine ALL
+        COMMENT "Building documentation"
+    )
+endif()