Add date format option in mapping doc

[abhidg]

Source date format option should be created in mapping doc to specify what date format to read in as the default. Output date format is always ISO8601, that is being tracked in #32

Alternatives

https://dateparser.readthedocs.io/en/latest/ does a good job of auto-parsing dates - but can be ambiguous with dates like 6/5/2024 which can be 6 May or 5 June depending on whether we interpret as MDY or DMY.