docs: write root README and architecture guide

[Copilot]

No root-level README.md existed for the DevSecNinja/.github repository, leaving the purpose and usage of its automation undocumented.

Adds a concise README.md with a navigation table linking to the key directories and a short development quick-start, plus a detailed docs/architecture.md covering:

• Repository layout — annotated directory tree
• Reusable workflows — each workflow documented with inputs table and minimal caller snippet (lint, release, config-sync, label-sync, labeler, todo-to-issue, assign-issue-to-codeowners)
• Workflow templates — summary table of all picker templates
• Config sync — table of auto-synced files; pointer to templates README
• Renovate configuration — table of shared preset fragments in .renovate/
• Design decisions — ADR index table linking into docs/design-decisions/

[DevSecNinja]

@copilot there is a lot of good info here but can you keep readme concise and move the rest to an architecture doc?

[Copilot]

@copilot there is a lot of good info here but can you keep readme concise and move the rest to an architecture doc?

Done in 2c753f7. README.md is now a short overview table + development quick-start. All the workflow input tables, example callers, config-sync file lists, Renovate preset details, and the ADR index have been moved to the new docs/architecture.md.