docs(collection): document all roles and fix metadata drift

Replace ansible-galaxy init placeholders across the collection and
correct documentation that drifted from the code, after a multi-agent
review of every role README against its defaults, tasks and templates.

Collection level:
- README: role table for all 16 roles, requirements and role-ordering
- galaxy.yml: declare community.docker and community.general deps,
  real description/tags/urls; normalize license to MIT-0
- meta/runtime.yml: requires_ansible '>=2.15.0'
- plugins/README: document the homarr_layout filter and
  garage_credentials lookup instead of scaffold boilerplate

Per-role meta/main.yml and README for the placeholder roles
(389ds, authentik, authentik_outpost_ldap, base, collabora, drawio,
garage, homarr, httpbin, keycloak, nextcloud, opencloud, traefik).

Correctness fixes found during review:
- keycloak: wrong domain default, drop invented keycloak_cert_resolver,
  document the provisioning feature
- garage: root_domain is .s3.<first-entry>, not the bare domain
- opnform: jwt/front_api secrets use `openssl rand -hex 32`; align the
  validation fail_msg in tasks/main.yml accordingly
- send: S3 example references garage_s3_domains[0] (was singular)
- opencloud: document required opencloud_wopi_domain

License normalized to MIT-0 across galaxy.yml, role meta and READMEs to
match the SPDX headers.

roles/nextcloud/meta/main.yml

@@ -0,0 +1,28 @@
+#SPDX-License-Identifier: MIT-0
+galaxy_info:
+  author: digitalboard
+  description: Deploy Nextcloud (fpm) + Redis + Postgres via Docker Compose behind Traefik
+  company: Digitalboard
+  license: MIT-0
+
+  min_ansible_version: "2.14"
+
+  platforms:
+    - name: Debian
+      versions:
+        - bookworm
+    - name: Ubuntu
+      versions:
+        - jammy
+        - noble
+
+  galaxy_tags:
+    - nextcloud
+    - files
+    - collabora
+    - oidc
+    - docker
+    - traefik
+    - digitalboard
+
+dependencies: []