Add NCISS decision points

[ahouseholder]

Creating this as a draft so we can figure out where to go with it.

Copilot Summary (replace as needed as the branch develops)

This pull request includes significant updates to the documentation and codebase related to the National Cybersecurity Incident Scoring System (NCISS) decision points. The most important changes include the addition of new documentation, the introduction of new decision points, and the refactoring of existing code to support these updates.

Documentation Updates:

• Added detailed documentation for Incident Severity, Observed Location of Activity, and Recoverability decision points in the docs/reference/decision_points/nciss/ directory. [1] [2] [3]
• Updated the mkdocs.yml configuration file to include the new NCISS decision points in the navigation.

Codebase Enhancements:

• Introduced new decision point classes and values for Incident Severity, Observed Location of Activity, and Recoverability in the src/ssvc/decision_points/nciss/ directory. [1] [2] [3]
• Added a new _Valued mixin class to the src/ssvc/_mixins.py file to support iteration over decision point values.
• Refactored the SsvcDecisionPoint class in src/ssvc/decision_points/base.py to use the new _Valued mixin and changed the values attribute to a tuple. [1] [2]

Miscellaneous:

• Added copyright and licensing information to the new files in the src/ssvc/decision_points/nciss/ directory. [1] [2]

These changes collectively enhance the documentation and functionality of the NCISS decision points within the SSVC framework.