Add a new document that precisely describes the string representation of
datetime and date types.
The previous description of them as "ISO 8601 Datetime" was very
imprecise, as ISO 8601 describes many different ways of representing a
datetime and date.
Specify the datetime format precisely as the ISO 8601 profile described
in RFC 3339, and the date format precisely as the "Complete date" format
from the W3C note on date and time formats.
Adjust all the references to ISO 8601 datetimes or dates to link back
to the definition.
Fixes#1420
* Clarify `group_key` for ungrouped notifications
* Clarify that `grouped_types` affects `group_key` but not streaming API
* Update content/en/methods/grouped_notifications.md
Co-authored-by: Claire <claire.github-309c@sitedethib.com>
---------
Co-authored-by: David Roetzel <david@roetzel.de>
