contributing+compose: clarify 'machine vierge' criterion for Jalon 0

Make explicit what was implicit: 'docker compose up' is not
expected to start everything on a bare host. Yavsc.Org (web)
requires an HTTPS signing certificate for IdentityServer8 in
Production mode, and the volume mount /etc/letsencrypt:/etc/letsencrypt:ro
is the documented way to supply it.

Two related changes:

- CONTRIBUTING.md, 'docker compose up' section: spell out that
  db + api + blogs start cleanly on a bare host, web fails with
  the documented IdentityServer error, and that the difference
  between 'vierge' and 'configured' is exactly the cert volume.
- docker-compose.yaml, web service: expand the commented volumes
  block to point at the same error message and reference the
  'HTTPS en production' section in CONTRIBUTING.md, so an
  operator reading the compose file knows what to uncomment
  and where to look.

docker-compose.yaml

@@ -56,6 +56,13 @@ services:
       - yavsc-internal
       - yavsc-public
     # volumes:
+    #   # Indispensable pour que Yavsc.Org (IdentityServer8) démarre.
+    #   # Sans ce montage, 'docker compose up' échoue côté web avec
+    #   # « Production IdentityServer requires a signing certificate ».
+    #   # Décommenter ici + côté api/blogs + renseigner
+    #   # Kestrel:Endpoints:Https:Certificate:{Path,KeyPath} dans
+    #   # appsettings-org.json (cf. section HTTPS en production dans
+    #   # CONTRIBUTING.md).
     #   - /etc/letsencrypt:/etc/letsencrypt:ro
 
   # ---------- Yavsc.Api (API REST principale) ----------