markup/goldmark: Make auto IDs GitHub compatible

You can turn off this behaviour:

```toml
[markup]
  [markup.goldmark]
    [markup.goldmark.parser]
      autoHeadingIDAsciiOnly = true
```
Note that the `anchorize` now adapts its behaviour depending on the default Markdown handler.

Fixes #6616

markup/goldmark/goldmark_config/config.go

@@ -69,6 +69,10 @@ type Parser struct {
 	// auto generated heading ids.
 	AutoHeadingID bool
 
+	// When AutoHeadingID is enabled this will generate IDs with Ascii
+	// characters only.
+	AutoHeadingIDAsciiOnly bool
+
 	// Enables custom attributes.
 	Attribute bool
 }