Write Stigcode user guide

[rdwj]

Acceptance Criteria:

• Getting started: first CKL export from a SARIF file in 3 commands
• Mapping database: how it works, confidence levels, how to contribute
• CKL export: options, profiles, incremental updates
• ATO evidence: report customization, template authoring
• Integration: piping from Sanicode, Semgrep, CodeQL, SonarQube
• Air-gapped deployment guide
• FAQ: common questions from ISSOs and assessors