Provide XSD schemas on GitHub pages

[DirkMahler]

The XML schemas from the Maven module core.schemata shall be published as part of the GH pages during the release builds. The resulting URLs shall be stable to be referenced in XML as schema locations , e.g.

• https://jqassistant.github.io/jqassistant/current/schema/jqassistant-plugin-v2.2.xsd
• https://jqassistant.github.io/jqassistant/current/schema/jqassistant-report-v2.3.xsd
• https://jqassistant.github.io/jqassistant/current/schema/jqassistant-rule-v2.2.xsd
• https://jqassistant.github.io/jqassistant/current/schema/jqassistant-baseline-v2.4.xsd