Docs: Add MD for the playbook-extension rule (#2561)

* Docs: Add MD for the playbook-extension rule

* chore: auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/ansiblelint/rules/playbook_extension.md

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+# playbook-extension
+
+This rule checks the file extension for playbooks is either `.yml` or `.yaml`.
+Ansible playbooks are expressed in YAML format with minimal syntax.
+
+The [YAML syntax](https://docs.ansible.com/ansible/latest/reference_appendices/YAMLSyntax.html#yaml-syntax) reference provides additional detail.
+
+## Problematic Code
+
+This rule is triggered if Ansible playbooks do not have a file extension or use an unsupported file extension such as `playbook.json` or `playbook.xml`.
+
+## Correct Code
+
+Save Ansible playbooks as valid YAML with the `.yml` or `.yaml` file extension.