Misc config loading fixes

The main motivation behind this is simplicity and correctnes, but the new small config library is also faster:

```
BenchmarkDefaultConfigProvider/Viper-16         	  252418	      4546 ns/op	    2720 B/op	      30 allocs/op
BenchmarkDefaultConfigProvider/Custom-16        	  450756	      2651 ns/op	    1008 B/op	       6 allocs/op
```

Fixes #8633
Fixes #8618
Fixes #8630
Updates #8591
Closes #6680
Closes #5192

config/docshelper.go

@@ -0,0 +1,45 @@
+// Copyright 2021 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/docshelper"
+)
+
+// This is is just some helpers used to create some JSON used in the Hugo docs.
+func init() {
+	docsProvider := func() docshelper.DocProvider {
+
+		cfg := New()
+		for _, configRoot := range ConfigRootKeys {
+			cfg.Set(configRoot, make(maps.Params))
+		}
+		lang := maps.Params{
+			"en": maps.Params{
+				"menus":  maps.Params{},
+				"params": maps.Params{},
+			},
+		}
+		cfg.Set("languages", lang)
+		cfg.SetDefaultMergeStrategy()
+
+		configHelpers := map[string]interface{}{
+			"mergeStrategy": cfg.Get(""),
+		}
+		return docshelper.DocProvider{"config": configHelpers}
+	}
+
+	docshelper.AddDocProviderFunc(docsProvider)
+}