demos/counter: inline gVisor walkthrough and fix external-volume, microVM, and 3-counter instructions #3492

demos/counter/README.md has several broken commands and layout issues:

1. Every command in the tutorial is hidden inside 16 collapsed <details> HTML blocks with MicroVM before GVisor (spelling gVisor as GVisor 8 times and missing "can" at line 41 "demo be run"). Inline the default gVisor walkthrough directly in the main steps (1. Build and Deploy, 2. Create a Counter Actor, 3. Proxy to the Actor, How to Use, Reaching a non-default port, and How to Uninstall) and move microVM and external CSI volume setup into dedicated optional sections at the end so the README renders cleanly on both GitHub and the docsite.
2. Lines 19-29: Document in Prerequisites that kubectl ate proxy actor requires atenet-router configured with --ingress-allowed-spiffe-ids=spiffe://cluster.local/ns/ate-system/sa/atenet-client (ATE_INGRESS_ALLOWED_SPIFFE_IDS), note that ATE_INSTALL_KIND=true sets SNAPSHOT_LOCATION automatically, and point the Quickstart links to ../../docs/get-started/quickstart.md instead of ../../README.md.
3. Lines 80-85 and 126: go run ./cmd/ate-setup deploy demo counter --with-external-volume --preview=ExternalVolumes fails unless ate-system was deployed with --preview=ExternalVolumes, an adapted CSI driver and CSIDriverConfig were installed (setup csi with --storage-class=csi-nfs-sc), and kubectl ate --preview create actor is used. Move this workflow into its own optional section with the required prerequisites.
4. Lines 56-69, 114, 138, 165, 182, 207, 231: The microVM setup commands omit ATE_CREDENTIAL_PROVIDER and ATE_INGRESS_ALLOWED_SPIFFE_IDS, reference hack/install-microvm-deps.sh without required --install, pass KIND_CLUSTER_NAME=<cluster> when hack/run-microvm-demo-kind.sh:42 already defaults KIND_CLUSTER_NAME to kind, and use my-counter-microvm-1 while hack/run-microvm-demo.sh:113 prints my-counter-1. Consolidate these steps into a single ## Running on microVM section.
5. Lines 15, 156-158, and 198-200: Prose says two counters are incremented, whereas demos/counter/counter.go:92-94,117 increments three (preserved memory count, preserved file counter, and preserved rootfs counter), and How to Use omits expected curl output.