Create a struct with all of Hugo's config options

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 #10896
Closes #10620

tpl/collections/index.go

@@ -64,7 +64,7 @@ func (ns *Namespace) doIndex(item any, args ...any) (any, error) {
 
 	lowerm, ok := item.(maps.Params)
 	if ok {
-		return lowerm.Get(cast.ToStringSlice(indices)...), nil
+		return lowerm.GetNested(cast.ToStringSlice(indices)...), nil
 	}
 
 	for _, i := range indices {