Add containers-pid-namespace compatibility flag (#28818)

This flag was added in cloudflare/workerd#6063 and the compatibility date added in cloudflare/workerd#6264

Author: gpanders

src/content/compatibility-flags/containers-pid-namespace.md

@@ -0,0 +1,16 @@
+---
+_build:
+  publishResources: false
+  render: never
+  list: never
+
+name: "Use an isolated PID namespace for containers"
+sort_date: "2026-04-01"
+enable_date: "2026-04-01"
+enable_flag: "containers_pid_namespace"
+disable_flag: "no_containers_pid_namespace"
+---
+
+When `containers_pid_namespace` is set, containers will use an isolated PID namespace. The `ENTRYPOINT` of your container will have PID 1.
+
+When unset, the container shares the PID namespace with the virtual machine (VM) containing the container. The `ENTRYPOINT` of your container will _not_ have PID 1 and other processes running on the VM (that are not part of your container) will be visible.