docs: OSS readiness — README hero, one-line installer, stale doc fixes

Restructure README to the standard OSS layout (logo, badges, screenshots
grid, architecture, quick start). Add install.sh for curl|sh installs with
a named-volume fallback for Docker Desktop file-sharing limits, and
scripts/setup.sh for clone-based setup. Correct SECURITY.md (no longer
pre-1.0) and CONTRIBUTING.md (auth shipped in 2.0.0).

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

SECURITY.md

@@ -7,7 +7,7 @@
 | latest release | yes |
 | older releases | no |
 
-Dash is pre-1.0; only the latest release receives fixes.
+Only the latest release receives fixes.
 
 ## Reporting a vulnerability