Fix help-osk nginx mount: serve site/ (Zensical default output, not public/)

zensical build outputs to site/ by default; nginx was mounting the empty
public/ and returning 403 (directory index forbidden). Point the web-server
mount, .gitignore and README at site/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

help-osk-docs/.gitignore

@@ -1,3 +1,3 @@
 # Build output — generated by `zensical build`, not tracked.
-/public/
+/site/
 .DS_Store