Prepare galaxy releasing

[afeefghannam89]

This adds the tooling for publishing and changelog. In addition, it fixes the metadata that would have broken the first publish.

Releasing

• changelogs/config.yaml plus a CI check that every pull request brings a fragment under
  changelogs/fragments/. Entries not worth telling users about go into trivial.
• .github/workflows/publish.yml, triggered manually with a tag. Builds the collection,
  publishes it to Galaxy and attaches the archive to the GitHub release. Manual because a Galaxy
  version cannot be replaced once uploaded.
• CONTRIBUTING.md, and RELEASE.md rewritten with the CLI fallback for whoever cannot trigger
  workflows.

galaxy.yml

• license: GPL-3.0 together with license_file would have failed the Galaxy import. The two
  are mutually exclusive and GPL-3.0 is a deprecated SPDX identifier. Now GPL-3.0-or-later.
• community.crypto was missing from dependencies although the beats role uses it.
• documentation was a 404, authors lost two people to a YAML indentation bug, tags had no
  auditbeat, and build_ignore let molecule and untracked local files into the archive.

Documentation

Restructured along the collection template. The multi-play example lived in the README and in
docs/getting-started.md and had drifted apart, it is in getting-started only now. Corrected the
elasticstack_release default, the broken indentation in the rsyslog example, and the claim that
OSS exists only for release 7, which holds for Elasticsearch and Kibana but not for Logstash and
Beats (#422). Module documentation is read from the modules instead of from copies under docs/.

Closes #521
Closes #422