Improve validation rule documentation for 2020-1

[martinpub]

There are XSLT stylesheets for generating HTML documentation from the Schematron and RelaxNG rulesets respectively. The output will be published on the Nordic Migrator web page for easy-access user reference.

There is room for improvement when it comes to presentation, e.g.:

• duplication of rule numbers
• unclear distinction between "Global" and "Detailed"
• filtering of rule number/name in nav-ncx schemadoc is flawed, resulting in e.g. "c_nav_ncx_1_a"
• etc.