Add XML file validator for all xml files

[wofsauge]

Idea is to add a small tool to the "XMLs" section of the docs, where you can paste your xml file and the tool checks if its valid in terms of its definition.
This check can be done by using an XML Syntax checker and additionally an XSD schema to compare the file against.

XSD file generator: https://www.freeformatter.com/xsd-generator.html

[wofsauge]

Tool is added now and is also available via Github actions