Track system mechanics concepts that have no corresponding `@Rule` support.

[aaclayton]

An evolving tracking issue for rules documentation gaps.

• "Difficulty Class" and "DC"
• Armor Types: Unarmored / Light / Medium / Heavy / etc...

[roth-michael]

• Item properties (e.g. "Bulky" for armor, "Thrown" for a weapon vs for an Action)
• Glancing Blows