docs/quadlet: ship unit files under lifecycle/quadlet/ + wire into dependabot and release workflows

Follow-up to #22454, which landed the Podman Quadlet install guide as docs-only. Two reviewer asks from that PR are deferred here because they go beyond documentation:

### Ship the unit files under `lifecycle/quadlet/`

The guide currently inlines the `.pod` / `.container` / `.volume` unit definitions. Extract them into `lifecycle/quadlet/` (mirroring how `lifecycle/container/` holds the compose file) and have the guide reference the committed files instead of inlining them.

### Wire the new directory into dependabot and the release workflows

- Add a dependabot entry for `lifecycle/quadlet/` so the pinned image tags get bumped like the rest.
- Hook the image tags into the release workflows so they bump alongside the compose file on each release.

Originally raised by @dominic-r on #22454 (the `lifecycle/quadlet/` placement and the dependabot/release-workflow comments).