Add basic support for generating API documentation using Doxygen

If CMake is run and Doxygen is found, add a make target called "doc",
which builds API documention in doc/html. This target is not included
in the default "all" target, you have to explicitly run "make doc" to
generate the documentation.

If graphviz is installed, in particular, if the "dot" binary is found,
doxygen is configured to generate various kinds of diagrams. Note that
due to this, the first run of doxygen can take a while.

.gitignore

@@ -26,6 +26,11 @@ tags
 minetest.conf
 debug.txt
 
+## Doxygen files
+doc/Doxyfile
+doc/html/
+doc/doxygen_*
+
 ## Build files
 CMakeFiles/*
 src/CMakeFiles/*
@@ -64,7 +69,7 @@ locale/
 *.layout
 *.o
 
-#build variants
+## Build variants
 build/android/assets
 build/android/bin
 build/android/Debug
@@ -75,5 +80,3 @@ build/android/libs
 build/android/obj
 timestamp
 
-
-