Add CVSS v4 DecisionTable docs for EQ1-6, Qualitative Severity Rating

[ahouseholder]

• resolves Add documentation for CVSS v4 DecisionTables #880
• extends DecisionTable to mermaid #886

and adds documentation pages for the six CVSS v4 EQ set DecisionTable objects added in

• Add DecisionTable objects for CVSS v4 Equivalence Sets #863
• Add CVSS v4 MacroVector to Quality Severity Rating DecisionTable #871

Copilot Summary

This pull request adds comprehensive documentation and navigation for modeling CVSS v4 Equivalence Sets and their Qualitative Severity Rating using SSVC decision tables. It introduces a new set of "how-to" documentation pages—one for each of the six CVSS v4 equivalence sets and one for the qualitative severity rating—explaining the analyst's workflow, decision points, outcomes, and providing executable examples and visualizations. The navigation is updated to include these new resources, and minor terminology corrections are made in the codebase for consistency.

Key changes:

Documentation: CVSS v4 Equivalence Sets and Qualitative Severity Rating

• Added new documentation pages for each CVSS v4 equivalence set (eq1.md through eq6.md) and for the qualitative severity rating (qualitative.md). These pages describe the analyst's workflow, decision points, outcomes, and include executable code blocks and decision model visualizations. [1] [2] [3] [4] [5] [6] [7]
• Added an overview page (index.md) explaining how SSVC can be used to model CVSS v4 assessments, the relationship between SSVC and CVSS, and how qualitative severity ratings are derived from equivalence sets.

Navigation Updates

• Updated the mkdocs.yml navigation to include a new "CVSS v4 Analyst Models" section, linking to all new documentation pages for easy access.

Terminology Consistency

• Fixed terminology in code and test helpers from "Equivalency Set" to "Equivalence Set" for accuracy and consistency with CVSS v4 specification. (qualitative_severity.py, _v4expected.py) [1] [2]

[ahouseholder]