Improve the documentation of logstash role

[afeefghannam89]

Migrates the logstash role's documentation to a single source of truth:
its meta/argument_specs.yml. Variable docs are no longer hand-maintained in
docs/role-logstash.md, instead Ansible validates the variables at runtime and
ansible-docsmith generates the
variable table in roles/logstash/README.md from the spec.

Part of #234, #218.

Changes

Documentation source & output

• roles/logstash/meta/argument_specs.yml — documents all role variables
  (incl. nested logstash_pipelines sub-options and output condition/congestion).
• roles/logstash/README.md — generated, compact variable table + hand-written
  intro, requirements, example, Tags section.
• .docsmith/readme.md.j2 — shared compact README template for all roles.
• roles/logstash/docs/pipelines.md — fixed examples to the correct list
  format and queue_type/queue_max_bytes; merged the duplicate
  docs/logstash-pipelines.md (removed).

Wiring & cleanup

• README.md / docs/getting-started.md — link to the role README.
• docs/role-logstash.md — slim pointer to the role README.
• README.md — Contributing note describing the argument_specs/README workflow.
• galaxy.yml — ignore .docsmith/ in the build.
• roles/logstash/defaults/main.yml — logstash_config_backup now an explicit
  boolean (false instead of no).
• Removed two unused variables: logstash_reset_writer_role (role/user management
  is handled by the netways.elasticstack.elasticsearch_role module) and
  logstash_queue_type (never wired into any template or any task)

CI

• .github/workflows/test_docs.yml — new docs gate (validate + README freshness).
• test_roles_pr.yml / test_role_logstash.yml — documentation-only changes no
  longer trigger the heavy molecule matrix.