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

resources/page/pages_language_merge.go

@@ -50,6 +50,7 @@ func (p1 Pages) MergeByLanguage(p2 Pages) Pages {
 
 // MergeByLanguageInterface is the generic version of MergeByLanguage. It
 // is here just so it can be called from the tpl package.
+// This is for internal use.
 func (p1 Pages) MergeByLanguageInterface(in any) (any, error) {
 	if in == nil {
 		return p1, nil