hugolib: Allow relative URLs in front matter

Before this commit you would have to do this in multilingual setups:

```
---
title: "Custom!"
url: "/jp/custom/foo"
---
```

This commit allows for relative URLs, e.g:

```
---
title: "Custom!"
url: "custom/foo"
---
```

Which is obviously easier and more portable.

The meaning of relative may change to include more in the future (e.g. role based access).

Fixes #5704

hugolib/site_render.go

@@ -208,7 +208,7 @@ func (s *Site) render404() error {
 		s:    s,
 		kind: kind404,
 		urlPaths: pagemeta.URLPath{
-			URL: path.Join(s.GetURLLanguageBasePath(), "404.html"),
+			URL: "404.html",
 		},
 	},
 		output.HTMLFormat,
@@ -271,7 +271,7 @@ func (s *Site) renderRobotsTXT() error {
 		s:    s,
 		kind: kindRobotsTXT,
 		urlPaths: pagemeta.URLPath{
-			URL: path.Join(s.GetURLLanguageBasePath(), "robots.txt"),
+			URL: "robots.txt",
 		},
 	},
 		output.RobotsTxtFormat)