[FEATURE-REQUEST] Custom page link placement

Amazing job on the custom page [https://github.com/aesmail/kaffy/issues/47], finally had a chance to try it out.

I feel that if a custom page is created within a resource it should appear inside the left menu of that resource.

```elixir
defmodule ChitChatWeb.RoomAdmin do
alias ChitChat.Accounts

def custom_pages(_schema, _conn) do
[
%{
slug: "my-own-thing",
name: "Secret Place",
view: ChitChatWeb.RoomView,
template: "custom-page.html",
assigns: [custom_message: "one two three"], # optional
order: 2 # optional
}
]
end
```

In other words if I create a custom page within `ChitChatWeb.RoomAdmin`, the link `Secret Place` should appear in Room section.