docs: add documentation for example 12 (daemon-job) and update quick start guide

[miguelgila]

Problem

examples/12-daemon-job/ is missing documentation and setup scripts that other examples have:

• No README.md — examples 10 and 11 have detailed READMEs explaining prerequisites, usage, and expected output
• No setup.sh — some examples (e.g., 04-volumes) include setup scripts for one-command deployment
• Not mentioned in quick start guide — docs/book/src/getting-started/quick-start.md should reference ReaperDaemonJob
• Not mentioned in documentation site — docs/book/src/reference/crds.md should document the ReaperDaemonJob CRD alongside ReaperPod and ReaperOverlay

What's needed

1. Add examples/12-daemon-job/README.md with:

   • Overview of ReaperDaemonJob concept
   • Prerequisites (CRD installed, controller running)
   • Step-by-step usage for both examples (simple + composable)
   • Expected output and verification commands

2. Add examples/12-daemon-job/setup.sh (optional) for one-command demo

3. Update docs/book/src/reference/crds.md:

   • Add ReaperDaemonJob CRD reference (spec fields, status fields, examples)
   • Document after dependency ordering
   • Document triggerOn, retryLimit, concurrencyPolicy

4. Update docs/book/src/getting-started/quick-start.md:

   • Mention ReaperDaemonJob as a way to run node-wide configuration tasks
   • Link to example 12

5. Update examples/README.md to include example 12 in the list