Add documentation on using HALE to configure GeoServer app-schema

[bgsmase]

May need to check whether this is still an alternative to new templating extension?

Sources of information:

• http://www.geo-solutions.it/blog/inspire-support-in-geoserver-made-easy-with-hale/
• http://www.dhpanel.eu/humboldt-framework/hale.html
• https://www.wetransform.to/downloads/ (link to GeoServer app-schema Plug-In page blank when tried 2021-09-03)
• https://geoserver.geo-solutions.it/edu/en/