docs(reference-ansible): add docs/ tree and document repo, playbooks, Makefile

Addresses the WKS PoC review (Notion 2026-05-26). All docs in English.
- README: purpose, docs table of contents, annotated repo tree
- docs/getting_started.md: prerequisites (WKS account, OIDC, SSH, VPN) + first deploy
- docs/ansible.md: playbook table, "Running Ansible", service parameters, cheatsheet
- docs/secrets.md: canonical Bao login (moved out of README) + demo defaults
- docs/operations.md: full Makefile reference
- docs/inventories.md: repo layout, topology, standard folder structure, walkthrough
- docs/testing.md: static checks, inventory resolution, smoke test / dry run
- remove ARCHITECTURE.md (architecture docs live externally)

Also includes the gymburgdorf inventory build-out (bookstack, homarr,
opnform, send) and scripts/bao-seed.sh. site.yml keeps a third traefik
play (traefik_servers minus the vagrant _dmz/_backend split) so the demo
inventories still configure their reverse proxy after the rebase onto main.

inventories/demo-gymburgdorf/hosts.yml

@@ -45,5 +45,21 @@ all:
         application:
 
     authentik_outpost_ldap_servers:
+      hosts:
+        application:
+
+    send_servers:
+      hosts:
+        application:
+
+    opnform_servers:
+      hosts:
+        application:
+
+    homarr_servers:
+      hosts:
+        application:
+
+    bookstack_servers:
       hosts:
         application: