[Docs] Document the processing order of sheets

[jfmcquade]

Add documentation (likely a flowchart) to explain the order of processing applied to sheets.

What I think would be most useful is trying to capture some documentation/flowchart on the processing order. I think it should actually be fairly straight-forward as the processing order is just something along the lines of

• Apply xlsx converter to extract flows from xlsx files.
• Apply default parser by flow, applying any @default or @row statements, convert _list and _collection columns, removes meta fields, handles translated fields, processes nested groups etc.
• Apply methods from extended parsers (e.g. data_list, data_pipe). These assign overrides per row, per flow, and across all processed flows

There is also a side flow that explains how caching and queues are used, but again reasonably straightforward (generally tracked by file md5 checksum, so that if a file contents are unchanged cached results used; exceptions possible, currently used for data_pipes which depend on child inputs so cache ignored)

_Originally posted by @chrismclarke in https://github.com/IDEMSInternational/parenting-app-ui/pull/1518#discussion_r982601174_

[esmeetewinkel]

This might be something Ed could get involved in