Create doc/guide for multiple domains hosting with Beacon (#393)

* Create doc/guide for multiple domains hosting with Beacon

* Update Multiple Domains Hosting guide

* Improve multiple_domains_hosting doc

guides/deployment/multiple_domains_hosting.md

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+# Multiple Domains/Tenants
+
+You can host multiple domains or subdomains with Phoenix by using the `:host` option of the scope function in your router:
+
+`:host` - a string or list of strings containing the host scope, or prefix host scope, ie "foo.bar.com", "foo."
+
+```elixir
+# match admin. subdomain
+scope "/", MyAppWeb, host: "admin." do
+  live "/", AdminLive, :new
+end
+
+# match example.com, and "example2.com"
+scope "/", MyAppWeb, host: ["example.com", "example2.com"] do
+  live "/", LandingLive, :new
+end
+
+# match my-example.org
+scope "/", MyAppWeb, host: "my-example.org" do
+  live "/", HomeLive, :new
+end
+```
+
+## Multiple Domains/Tenants hosting with BeaconCMS
+
+So if you need to host multiple domains or subdomains with Beacon, you can use the same `:host` option:
+
+Here's another example, but now using Beacon:
+
+```elixir
+  # serve the `:demo` site at demo.org/demo
+  scope "/", host: "demo.org" do
+    pipe_through :browser
+
+    beacon_site "/demo", site: :demo
+  end
+
+  # serve the `:blog` site at blog.com
+  scope "/", host: "blog.com" do
+    pipe_through :browser
+
+    beacon_site "/", site: :blog
+  end
+
+  # serve the admin interface at the prefix /admin on the root domain
+  scope "/admin" do
+    pipe_through :browser
+    beacon_live_admin "/"
+  end
+```
+
+You also need to pass the `:check_origin` option when configuring your
+endpoint explicitly outlining which origins are allowed:
+
+Edit `config/runtime.exs` and edit the following config inside `config :my_app, MyAppWeb.Endpoint, ...`:
+
+```elixir
+    check_origin: [
+      "https://beacon-demo.com/",
+      "https://demo.org",
+      "https://blog.com"
+    ],
+```