Rename doc subdirectory to docs for higher consistency with OSS standards

docs/CMakeLists.txt

@@ -0,0 +1,25 @@
+# Building doxygen documentation
+
+find_package(Doxygen)
+
+if(BUILD_DOXYGEN_DOC)
+    if(DOXYGEN_FOUND)
+        configure_file(${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile.in ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile @ONLY)
+
+        add_custom_target(doc
+            COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
+            WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+            COMMENT "Generating API documentation with Doxygen"
+            VERBATIM)
+
+        install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/html DESTINATION ${CMAKE_INSTALL_DOCDIR} OPTIONAL)
+    else()
+        message(WARNING "Documentation enabled, but Doxygen cannot be found")
+    endif()
+endif()
+
+install(FILES sdbus-c++-class-diagram.png
+              sdbus-c++-class-diagram.uml
+              systemd-dbus-config.md
+              using-sdbus-c++.md
+        DESTINATION ${CMAKE_INSTALL_DOCDIR})