Primary motivation is documentation, but it will also hopefully simplify the code.
Also,
* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.
Closes#10896Closes#10620
Note that this has only been a problem with "raw dates" in TOML files in /data and similar. The predefined front matter
dates `.Date` etc. are converted to a Go Time and has worked fine even after upgrading to v2 of the go-toml lib.
Fixes#9979
`go-toml/v2`'s unmarshaler does not specify zone name even if value has
offset explicitly.
To make time-formatting behaviour consistent, convert them into string
in hugo.
Close#8895
