docs: simplify README and define wiki boundary

[icidade]

Summary

• simplify the main README into a landing-page style entry point
• add an explicit README vs docs vs Wiki boundary document
• point readers toward the wiki for editorial/explanatory content

What changed

• reduced README detail for profiles, self-update, architecture explanation, and external finding workflow
• grouped documentation links into higher-signal navigation buckets
• added docs/readme-docs-wiki-boundary.md to formalize the content split

Validation

• ./scripts/validate-repo.sh
• local link validation for README and wiki draft
• local review commit for wiki draft (push still blocked because .wiki.git is not materialized)

Wiki status

The public wiki content was prepared locally, but push to https://github.com/icidade/miniCISO.wiki.git still fails with Repository not found, which suggests the wiki Git backend has not been materialized yet.