DRAFT: Update reporting templates

This MR makes some changes to the reporting templates:

Threat Briefing Template:

• Removes the "Threat Landscape Overview" section. This detracts from the highlighted threats. We can offer another service that is a general landscape update, probably on a quarterly basis.
• Turns the highlighted threats into individual collapsable sections. This gives better visibility to the threats, especially for senior leadership who will want to see the highlights straight away.
• Moves references into individual threat sections - this makes it easier to see how the references align with the specific threats.

Both reporting templates:

• Uses "Low/Medium/High" terminology as a standard, instead of "Poor/Fair/Good" to move away from terms related to blame

Also, collapsable sections requires inline HTML so I had to update the markdownlint rules to allow it.