[Volunteer Engagement]Create architecture document

Many volunteers join our team and stuck on setting up environment and learning about our system.

It blocks volunteers to contribute to the project. They need more support from the team.

From the team's aspect, it is hard to guide each volunteer through the onboarding process.

More documents could help to shorten onboarding process and encourage contributions. Also, an architecture document that provides a whole picture of the project would be extremely helpful.