Update docs for os-healthcheck toolkit layout

infra-run/scripts/bash/os-healthcheck/disk_check.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -o errexit
+set -o nounset
+set -o pipefail
+
+threshold="${1:-80}"
+
+if [[ ! "$threshold" =~ ^[0-9]+$ ]] || (( threshold < 1 || threshold > 100 )); then
+  printf 'CRITICAL: invalid threshold "%s"; provide an integer from 1 to 100\n' "$threshold" >&2
+  exit 2
+fi
+
+status=0
+warning_threshold=$(( threshold > 5 ? threshold - 5 : threshold ))
+
+while read -r filesystem size used avail use_percent mountpoint; do
+  usage="${use_percent%\%}"
+
+  if (( usage >= threshold )); then
+    printf 'CRITICAL: %s mounted on %s is %s used; threshold is %s%% (%s free)\n' "$filesystem" "$mountpoint" "$use_percent" "$threshold" "$avail"
+    status=1
+  elif (( usage >= warning_threshold )); then
+    printf 'WARNING: %s mounted on %s is %s used; threshold is %s%%\n' "$filesystem" "$mountpoint" "$use_percent" "$threshold"
+  else
+    printf 'OK: %s mounted on %s is %s used\n' "$filesystem" "$mountpoint" "$use_percent"
+  fi
+done < <(df -P -x tmpfs -x devtmpfs | awk 'NR > 1 {print $1, $2, $3, $4, $5, $6}')
+
+exit "$status"