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/garage/README.md

@@ -20,8 +20,10 @@ common overrides:
 
 ### Service
 
-- `garage_s3_domains`: FQDNs the S3 router accepts. First entry is the
-  canonical hostname and is used as `root_domain` in `garage.toml`.
+- `garage_s3_domains`: FQDNs the S3 router accepts. The first entry is the
+  canonical hostname; `garage.toml` derives the virtual-hosted-style S3
+  `root_domain` from it as `.s3.<first-entry>` (so buckets resolve under
+  `<bucket>.s3.<first-entry>`).
 - `garage_web_domain`, `garage_webui_domain`: separate hostnames for
   the S3-website endpoint and the console.
 - `garage_image`, `garage_replication_factor`, `garage_db_engine`,