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

hugolib/pages_process.go

@@ -199,6 +199,5 @@ func (p *sitePagesProcessor) doProcess(item any) error {
 }
 
 func (p *sitePagesProcessor) shouldSkip(fim hugofs.FileMetaInfo) bool {
-	// TODO(ep) unify
-	return p.m.s.SourceSpec.DisabledLanguages[fim.Meta().Lang]
+	return p.m.s.conf.IsLangDisabled(fim.Meta().Lang)
 }