docs(runbooks): add "new web app" setup runbook under doc/runbooks/

Document, as a tree-docs tree, the end-to-end procedure to stand up a new
web application on the Arcodange platform — a mechanic spread across the
factory, tools and app repos with non-trivial ordering dependencies.

Covers: Gitea repo creation (org-secret inheritance), Postgres DB + owner
role (factory/postgres/iac), platform Vault declaration (gitea_cicd_<app>
+ policies, tools/hashicorp-vault/iac), the app Helm chart (VSO dynamic
secrets via pgbouncer), the app Terraform (app_roles module), the CI
workflows (tofu apply + image build, incl. the copy-pasted role pitfall),
and ArgoCD registration (factory/argocd/values.yaml). Adds a naming-
conventions concept page and an ordered checklist.

Wires the legacy doc/adr "setup hello world web app" item and the factory
README to the runbook. New docs live under doc/ (singular) per the PR #8
convention.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

doc/adr/README.md

@@ -18,9 +18,9 @@
   - [ ] terrakube
   - [ ] prometheus/grafana
   - [ ] ansible AWX
-- [ ] setup hello world web app
-  - [ ] manage postgres credentials
-  - [ ] protect public endpoint (crowdsec)
+- [ ] setup hello world web app — 📖 procédure complète : [runbook « Nouvelle application web »](../runbooks/new-web-app/README.md)
+  - [ ] manage postgres credentials → [base de données](../runbooks/new-web-app/02-database.md) + [Vault plateforme](../runbooks/new-web-app/03-vault-platform.md)
+  - [ ] protect public endpoint (crowdsec) → [chart : ingress public](../runbooks/new-web-app/04-helm-chart.md)
 
 > [!NOTE]
 > Reference: [Arcodange _**Factory**_ Ansible Collection](/ansible/arcodange/factory/README.md)