search: make it possible to disable structural search

Created by: rvantonder

Adds this option to site config:

  "experimentalFeatures": {
    "structuralSearch": "disable"
  },

I am enabling it by default, with the option to disable. Can someone verify that "default": "enabled" in schema/site.schema.json is enough to turn this on by default, or whether a additional configuration files need to change?

The notification is red, thoughts on whether this should be a warning (blue) instead?