doxygen: add build-tools tag (#34249)

* doxygen: add build-tool tag
   This allows it to be included automatically as an external. No one links against doxygen so this should be ok.
* doxygen: add self as maintainer

var/spack/repos/builtin/packages/doxygen/package.py

@@ -39,8 +39,12 @@ class Doxygen(CMakePackage):
 
     variant("mscgen", default=False, description="Build with support for code graphs from mscgen.")
 
+    tags = ["build-tools"]
+
     executables = ["doxygen"]
 
+    maintainers = ["sethrj"]
+
     @classmethod
     def determine_version(cls, exe):
         output = Executable(exe)("-v", output=str, error=str)