Add section about date columns

[zkamvar]

A recent bug report in #118 where folks submitting datetime columns where they should have been submitting dates was causing an execution error (fixed in hubverse-org/hubValidations#119) prompted me to notice that:

• Apart from the schema for specific task IDs we don't really mention that we currently only support dates, specifically in ISO format and not datetimes across the hubverse.
• There is also nothing stopping admins from adding their own custom taskIDs where they might expect data times (although it's hard to imagine how they would encode datetime valid values in the config). In any case, our framework currently cannot support handling of datetimes. This would require us to add mappings between json datetime data type to R datetime datatype and finally to the arrow datetime datatype equivalent.

So my questions are:

1. Should we add a specific section on dates somewhere in hubDocs? Any preferences for where?
2. Are datetimes sth we should support? If not, should we make that clear in the aforementioned docs section?

Originally posted by @annakrystalli in hubverse-org/.github#31

The initial thought in the meeting was to documentation that we specify only accepting dates for now in the documentation.

Possibly in:

• this repository
• hubValidations
• hubData (maybe)

Willing to hear about other options.