Add FuncValueConveter Sample

[timunie]

What does the pull request do?

Adds a missing sample about FuncValueConverter usage

Scope of this PR:

• [x] fix or update to an existing sample
• [ ] add a new sample

What is the current behavior?

Only IValueConverter and IMultiValtueConverter was covered

Checklist

If this is a new Sample

• [ ] Added a ReadMe-file
• [ ] Updated the landing page
• [x] Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with a link to your sample in the related documentation page --> to be checked

In any case

• [x] Spell-checking done
• [x] Checked if all hyperlinks work
• [x] Checked if all images are visible

Fixed issues

Fixes #51

[timunie]

FuncValueConverter is yet missing from the docs. Will add it once this is merged.

draft:

## FuncValueConverter

You can also implement a `FuncValueConverter` if you don't need to convert back and also not the the `ConverterParameter`. The FuncValueConverter has two generic parameters:

**TIn**: This parameter defines the expected input type. This can also be an array in case you want to use this converter in a MultiBinding.

**TOut**: This parameter defines the expected output type.

### Example:
```cs
public static FuncValueConverter<string, string> MyConverter { get; } = new FuncValueConverter<string, string>(s => $"You said '{s}');

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