Add guide to docs for managing firewall rules across multiple sites

templates/guides/multiple-site-firewall.md.tmpl

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+---
+subcategory: ""
+page_title: "Manage Firewall Rules for Multiple Sites - Unifi Provider"
+description: |-
+    An example of applying firewall rules to multiple sites.
+---
+
+# Manage Firewall Rules for Multiple Sites
+
+The provider takes a default site value but all resources in the provider should allow overriding of the
+site you are managing. In order to apply and manage a firewall rule across multiple sites, you simply
+need to provide different values for the `site` attribute to `unifi_firewall_rule`:
+
+{{ tffile "examples/multiple_site_firewall/firewall.tf" }}
+
+You could optionally load lists of sites from JSON/CSV, variables, or other sources.
+
+When you apply this configuration it will create the same firewall rule on every site in the list.
+If you need to update the rule, you simply make an update to the rule definition and Terraform will
+apply/update it across all the sites. If you add / or remove a site from the list, Terraform will also
+handle creating or removing the rule on the subsequent `terraform apply`.