Validator should warn about raw HTML content in markdown

[mmsmits]

Clarification about R5 about the markdown data type:

Processors SHALL treat embedded XML tags as string content, not as tags. This may be done by pre-processing and escaping any ">" characters preceding character content (">" becomes ">") before processing the content or by using a markdown processor-specific flag that accomplishes the same effect. This means that HTML content cannot be embedded in markdown to influence rendering.

The validator should enforce that no raw html is inside the markdown. The Java validator returns a warning in this case.

[mmsmits]

Moved from VONK-4520