Update validation rules based on feedback: content files

[josteinaj]

Schematron: https://github.com/nlbdev/nordic-epub3-dtbook-migrator/blob/guidelines-revision/src/main/resources/xml/schema/2020-1/nordic2020-1.sch

• Update nordic2020-1.sch based on feedback from the guidelines group: https://github.com/nlbdev/epub3-guidelines-update/issues/82
• Find out which, if any, of the validation rules are already covered by epubcheck