Don't automatically build docs

This means you can leave the docs enabled in the build without being
constantly spammed by all the console output that entails.

https://salsa.debian.org/games-team/libsfml/-/blob/master/debian/patches/02_build-doc-once.patch

doc/CMakeLists.txt

@@ -45,7 +45,7 @@ configure_file(${DOXYGEN_INPUT_DIR}/doc/header.html.in ${DOXYGEN_OUTPUT_DIR}/hea
 configure_file(${DOXYGEN_INPUT_DIR}/doc/doxygen.css ${DOXYGEN_OUTPUT_DIR}/html/doxygen.css COPYONLY)
 
 # target setup
-add_custom_target(doc ALL
+add_custom_target(doc
                   COMMAND ${CMAKE_COMMAND} -E echo_append "Building API Documentation..."
                   COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYGEN_CONFIGURED_INPUT}
                   COMMAND ${CMAKE_COMMAND} -E echo "Done."