[S7] Write user-facing documentation

[krakenhavoc]

Description

Documentation for users setting up and using the GitHub integration.

Documents

1. Setup Guide — how to install the GitHub App, configure it, create your first .krakenkey.yml
2. Config Reference — complete .krakenkey.yml schema with all options documented and examples
3. GitHub Action Reference — all commands, inputs, outputs with workflow examples
4. Troubleshooting Guide — common issues, error messages, and fixes
5. OIDC Setup Guide — keyless auth configuration

Acceptance Criteria

• All 5 documents written
• Config reference covers every field with examples
• Action reference has copy-paste workflows
• Troubleshooting covers top 10 expected issues
• Published to krakenkey.io/docs