cmake: Add docs build target

doc/CMakeLists.txt

@@ -0,0 +1,25 @@
+# Copyright (c) 2024-present The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://opensource.org/license/mit/.
+
+find_package(Doxygen COMPONENTS dot)
+
+if(DOXYGEN_FOUND)
+  set(doxyfile ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile)
+  configure_file(Doxyfile.in ${doxyfile})
+
+  # In CMake 3.27, The FindDoxygen module's doxygen_add_docs()
+  # command gained a CONFIG_FILE option to specify a custom doxygen
+  # configuration file.
+  # TODO: Consider using it.
+  add_custom_target(docs
+    COMMAND Doxygen::doxygen ${doxyfile}
+    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+    COMMENT "Generating developer documentation"
+    VERBATIM USES_TERMINAL
+  )
+else()
+  add_custom_target(docs
+    COMMAND ${CMAKE_COMMAND} -E echo "Error: Doxygen not found"
+  )
+endif()