demos/{sandbox,jupyter,multi-template}: add ingress prerequisite, fix Quickstart links, CLI states, and counter output #3494

Shared across demos/{sandbox,jupyter,multi-template}/README.md

1. Document --ingress-allowed-spiffe-ids=spiffe://cluster.local/ns/ate-system/sa/atenet-client (ATE_INGRESS_ALLOWED_SPIFFE_IDS) and KO_DOCKER_REPO (or --config / --image-repo and --image-tag) in Prerequisites (noting that ATE_INSTALL_KIND=true sets SNAPSHOT_LOCATION and KO_DOCKER_REPO automatically on kind), and replace [Quickstart](../../README.md#...) links with ../../docs/get-started/quickstart.md and ../../docs/guides/operator-install.md.

demos/sandbox/README.md
2. Lines 76-93 show only interactive sandbox> stdin prompts. Add a non-interactive one-liner (printf 'echo "Hello" > test.txt\ncat test.txt\nexit\n' | ./bin/sandbox-client ...) so scripts and coding agents do not hang on stdin.
3. Clarify at lines 13 and 22 that each /process call spawns a fresh sh -c process (resetting cd and shell variables) while container filesystem and memory persist across requests and snapshots, fix "an sandboxed" (line 15), and remove the duplicate go install ./cmd/kubectl-ate (line 61).

demos/jupyter/README.md
4. Replace STATUS_SUSPENDED (line 79) with ACTOR_STATE_SUSPENDED (api/ateapi/v1/ateapi.proto:876-885), remove the unimplemented "suspends when idle" claims (lines 8, 13 and cmd/ate-setup/internal/demos/jupyter/jupyter.go:16), fix step numbering (### 1., ### 2., ### 4. skipping 3), promote Clean up to H2 including WorkerPool and namespace deletion, write quay.io/jupyter/base-notebook, and add a curl -s http://localhost:8888/api verification step (note: HEAD /lab via curl -I returns 405 Method Not Allowed).

demos/multi-template/README.md and counter-template.yaml.tmpl
5. README.md:74-75 shows # -> hello from: <ip> | preserved memory count: 1, whereas demos/counter/counter.go:117 prints ... | preserved file counter: -1 | preserved rootfs counter: 1 because counter-template.yaml.tmpl:25-36 omits the /home/counter durableDir mount. Mount /home/counter and update the expected output, drop the retired DNS routing claim (line 48), and stop background kubectl ate proxy actor processes (lines 64-65) in cleanup.