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/resource_factories/create/create.go

@@ -136,6 +136,7 @@ func (c *Client) match(name, pattern string, matchFunc func(r resource.Resource)
 }
 
 // FromString creates a new Resource from a string with the given relative target path.
+// TODO(bep) see #10912; we currently emit a warning for this config scenario.
 func (c *Client) FromString(targetPath, content string) (resource.Resource, error) {
 	return c.rs.ResourceCache.GetOrCreate(path.Join(resources.CACHE_OTHER, targetPath), func() (resource.Resource, error) {
 		return c.rs.New(