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/opnform/tasks/main.yml

@@ -15,10 +15,11 @@
       - opnform_front_api_secret | length > 0
       - opnform_db_password | length > 0
     fail_msg: >-
-      OpnForm requires opnform_app_key (prefix 'base64:'), opnform_jwt_secret,
+      OpnForm requires opnform_app_key, opnform_jwt_secret,
       opnform_front_api_secret and opnform_db_password.
-      Generate with: openssl rand -base64 32
-      The app_key MUST be prefixed with "base64:"
+      Generate with:
+      opnform_app_key='base64:'$(openssl rand -base64 32) (the 'base64:' prefix is required);
+      opnform_jwt_secret and opnform_front_api_secret via openssl rand -hex 32.
       Provide via OpenBao, Ansible Vault or extra-vars.
     success_msg: Secrets validation passed