Update copyright notices in all docs

[blingdahl]

Current notice:

/**
 * Copyright 2022 Google LLC
 *
 * 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.
 */

New notice: TBD

[caravansteve]

For the new notice, let's do the following:

1) Change the copyright notice to: Copyright 2023 The Open Product Recovery Authors

2) Create a AUTHORS file in the root directory of the repo.

3) Make sure "Google LLC" is listed as a contributor in the AUTHORS file, and also add any key contributors who have been contributing to this project in an individual capacity since its transfer from Google.