Disallow foreign namespaces

[SteveLLamb]

• Update documentation about conversion tools and danger of introducing external/foreign namespaces, and disallow them.
• Update validation to reject, prior to build (live previews) - right now will reject in build when pushed to repo!

[palemieux]

Probably need to allow:

• svg
• mathml
• foreign namespaces on metadata items

[SteveLLamb]

Example of build error:
"file:/home/runner/work/rdd54-private/rdd54-private/doc/main.html":366.68-366.120: error: Attribute "xmlns:ns0" not allowed here

From:
<math xmlns:ns0="http://www.w3.org/1998/Math/MathML">