[OTHER] Add CLA and CLA Sign Workflow

[flavioislima]

This adds a CLA file under doc/cla.md
Also adds a github action to trigger to all new contributors to sign the CLA. It will be stored under signatures/version1/cla.json.

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)