Add support for a content dir set per language

A sample config:

```toml
defaultContentLanguage = "en"
defaultContentLanguageInSubdir = true

[Languages]
[Languages.en]
weight = 10
title = "In English"
languageName = "English"
contentDir = "content/english"

[Languages.nn]
weight = 20
title = "På Norsk"
languageName = "Norsk"
contentDir = "content/norwegian"
```

The value of `contentDir` can be any valid path, even absolute path references. The only restriction is that the content dirs cannot overlap.

The content files will be assigned a language by

1. The placement: `content/norwegian/post/my-post.md` will be read as Norwegian content.
2. The filename: `content/english/post/my-post.nn.md` will be read as Norwegian even if it lives in the English content folder.

The content directories will be merged into a big virtual filesystem with one simple rule: The most specific language file will win.
This means that if both `content/norwegian/post/my-post.md` and `content/english/post/my-post.nn.md` exists, they will be considered duplicates and the version inside `content/norwegian` will win.

Note that translations will be automatically assigned by Hugo by the content file's relative placement, so `content/norwegian/post/my-post.md` will be a translation of `content/english/post/my-post.md`.

If this does not work for you, you can connect the translations together by setting a `translationKey` in the content files' front matter.

Fixes #4523
Fixes #4552
Fixes #4553

create/content.go

@@ -63,7 +63,22 @@ func NewContent(
 		return err
 	}
 
-	contentPath := s.PathSpec.AbsPathify(filepath.Join(s.Cfg.GetString("contentDir"), targetPath))
+	// The site may have multiple content dirs, and we currently do not know which contentDir the
+	// user wants to create this content in. We should improve on this, but we start by testing if the
+	// provided path points to an existing dir. If so, use it as is.
+	var contentPath string
+	var exists bool
+	targetDir := filepath.Dir(targetPath)
+
+	if targetDir != "" && targetDir != "." {
+		exists, _ = helpers.Exists(targetDir, ps.Fs.Source)
+	}
+
+	if exists {
+		contentPath = targetPath
+	} else {
+		contentPath = s.PathSpec.AbsPathify(filepath.Join(s.Cfg.GetString("contentDir"), targetPath))
+	}
 
 	if err := helpers.SafeWriteToDisk(contentPath, bytes.NewReader(content), s.Fs.Source); err != nil {
 		return err