[Doc] Add eng docs about our data sync flow

[jcqli]

Will want to include these docs as part of https://github.com/google/ground-platform/issues/1722

Requirements:

1. Data Flow / Pipeline of Data Sync Process
2. Mention each stage of the Data Sync Process
3. Failure Cases of Data Sync Process
4. Happy Cases of Data Sync Process

[jcqli]

@anandwana001 Could you add some more detail on what's needed here (as per discussion in standup today)