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
And we have taken great measures to limit potential site breakage:
* For `disableKinds` and `outputs` we try to map from old to new values if possible, if not we print an ERROR that can be toggled off if not relevant.
* The layout lookup is mostly compatible with more options for the new `term` kind.
That leaves:
* Where queries in site.Pages using taxonomy/taxonomyTerm Kind values as filter.
* Other places where these kind value are used in the templates (classes etc.)
Fixes#6911Fixes#7395
