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/authentik/README.md

@@ -101,8 +101,13 @@ from the list to keep state clean.
 
 ## Dependencies
 
-- Traefik network (`authentik_traefik_network`, default `proxy`)
-- Internal backend network (`authentik_backend_network`, default `backend`)
+- Run `digitalboard.core.base` first (Docker) and have the `community.docker`
+  collection installed; the role drives the stack via
+  `community.docker.docker_compose_v2`.
+- Traefik network (`authentik_traefik_network`, default `proxy`) must exist
+  beforehand (e.g. created by the traefik role); it is referenced as an
+  external network in the Compose file.
+- Internal backend network (`authentik_backend_network`, default `backend`).
 
 ## Example playbook