docs: centralize Mimir documentation

[jb-thery]

Summary

• make the root README the canonical Mimir product documentation
• reduce package README files to minimal npm entrypoints with prominent GitHub documentation links
• update npm homepage metadata and prepare version 0.4.3 so npm displays the corrected package README files after publish

Validation

• pnpm validate
• git diff --check
• inspected generated 0.4.3 tarballs to confirm package README files include the GitHub documentation link