chore: Consolidate SearchEngines policy into single page

[bsmth]

Description:

Testing with the following example:

{
  "policies": {
    "SearchEngines": {
      "Add": [
        {
          "Name": "Example1",
          "URLTemplate": "https://www.example.org/q={searchTerms}",
          "Method": "GET",
          "IconURL": "https://www.example.org/favicon.ico",
          "Alias": "example",
          "Description": "Description",
          "PostData": "name=value&q={searchTerms}",
          "SuggestURLTemplate": "https://www.example.org/suggestions/q={searchTerms}"
        }
      ],
      "Default": "Perplexity",
      "PreventInstalls": true,
      "Remove": ["Ecosia"]
    }
  }
}

Although noting PreventInstalls doesn't seem to be applying either on its own or as part of a larger example like this. Should confirm expectations about this one or if we need a bug to track it.

Motivation:

We currently have four pages for the SearchEngines policy, although it appears the options are not mutually exclusive.

Related issues and pull requests:

Exists from initial import based on https://mozilla.github.io/policy-templates/

Note: we could also do the same for Certificates and SanitizeOnShutdown.