Need to allow more complex content in `remarks` and `propulsion`

[IanMayo]

We've started processing more complex HTML content. I think we're generating legitimate DITA content, but it is failing against the schema.

Notes:

• we don't have headings in DITA. So, I'm happy to use a <p outputclass="h2"> instead
• we're converting <div> entries to <span>

I've inserted the new content into this file: https://github.com/DeepBlueCLtd/Fi3ldMan/blob/main/dita/regions/britain/hybrid/unit_hybrid_a.dita

We need XML Schema changes so that the file is valid in OxygenXML/DITA_OT.

The output of this issue is a modified Schema(s) for the class element in the dita/dtd folder of this repo.

[IanMayo]

This is now done, since we switched those elements to be based upon section