Fix up devcontainer config (REUSE, strict JSON, image bump) and mention it in docs

- add SPDX licensing information for REUSE compliance
- drop the JSONC comment from devcontainer.json, keeping it strict JSON
  (as required by the check-json pre-commit hook)
- bump the base image to the current ghcr.io/devture/ansible release
- mention the dev container in docs/ansible.md

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.devcontainer/devcontainer.json.license

@@ -0,0 +1,3 @@
+SPDX-FileCopyrightText: 2025 Jonah Aragon
+
+SPDX-License-Identifier: AGPL-3.0-or-later