doc: Add documentation-generating scripts to the Crashpad tree

doc/generate_doxygen.sh

+#!/bin/sh
+
 # Copyright 2015 The Crashpad Authors. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
 #
 #     http://www.apache.org/licenses/LICENSE-2.0
 #
 # Unless required by applicable law or agreed to in writing, software
 # distributed under the License is distributed on an "AS IS" BASIS,
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-This file is used by Paths::TestDataRoot() to locate the test data root. It
-is present at a known path from the test data root.
+set -e
+
+# Generating Doxygen documentation requires Doxygen, http://www.doxygen.org/.
+
+# Run from the Crashpad project root directory.
+cd "$(dirname "${0}")/.."
+
+output_dir=out/doc/doxygen
+
+rm -rf "${output_dir}"
+mkdir -p "${output_dir}"
+
+doxygen doc/crashpad.doxy