[Documentation]: Rework the whole collection documentation

[afeefghannam89]

Goal

Move every role's variable documentation to a single source of truth: each
role's meta/argument_specs.yml. This gives runtime input validation and
feeds an auto-generated variable table in the role's README.md, rendered with
ansible-docsmith. A
Test Documentation CI workflow fails a pull request if a README drifts from
its spec.

This replaces the hand-maintained variable lists in docs/role-*.md, which
could silently drift from defaults/main.yml.

This is part of #234 and #218

Approach — one PR per role

For each role:

1. Add meta/argument_specs.yml covering every variable in defaults/main.yml.
2. Add a README.md with the docsmith markers + generated variable table:
3. Point README.md, docs/getting-started.md and docs/role-<role>.md at the
   new role README.

Shared infrastructure (.docsmith/readme.md.j2, the Test Documentation gate,
and the contributor guide in the main README) landed with the first role.

Roles

• logstash — Improve the documentation of logstash role #473
• repos Rework repos role documentations #482
• beats Rework the beats role documentations #480
• kibana
• elasticsearch — closes Documentation about management of paths for Elasticsearch is confusing #218
• elasticstack (shared elasticstack_* variables)
• Central documentation page for shared Central documentation page for shared elasticstack_* variables #475

Done when

All boxes are checked → this closes #234 and #218.