Loading, saving and loading RTF leads to exception

### Description

Loading some RTF in a `RichTextBox`, saving it and loading it again causes an exception to be thrown.

This happens because the RTF contains a hyperlink which contains an umlaut ("ö" in this example).
That umlaut is correctly escaped in the source RTF with "\'f6".
Loading the source RTF works fine.
Saving the RTF with `RichTextBox` causes the correctly escaped text to become `'f6`.
Loading this again causes an exception while trying to construct the NavigationUri, as an unescaped "'" is not valid in an URI.

After some investigation the issue seems to be that:
XamlToRtfWriter.WriteInlineChild calls BamlResourceContentUtil.UnescapeString which then simply removes the "\\" before the "'", thus producing an invalid URI.

One way to solve this would be to add
```
else if (documentNode.NavigateUri[i] == "'")
{
_rtfBuilder.Append("\\'");
}
```

### Reproduction Steps

See attached repro project.
[UmlautIssue.zip](https://github.com/dotnet/wpf/files/10918031/UmlautIssue.zip)

### Expected behavior

No exception should occur and RTF should be rendered.

### Actual behavior

Exception is thrown during load (after save from `RichTextBox`).

### Regression?

_No response_

### Known Workarounds

_No response_

### Impact

_No response_

### Configuration

Tested with .NET 4.8 and .NET 6.0.

### Other information

_No response_