Add provider specifying example to rails guide (#233)

## What this does

Provide similar example that answered #226

## Type of change

- [ ] Bug fix
- [ ] New feature
- [ ] Breaking change
- [X] Documentation
- [ ] Performance improvement

## Scope check

- [X] I read the [Contributing
Guide](https://github.com/crmne/ruby_llm/blob/main/CONTRIBUTING.md)
- [X] This aligns with RubyLLM's focus on **LLM communication**
- [X] This isn't application-specific logic that belongs in user code
- [X] This benefits most users, not just my specific use case

## Quality check

- [X] I ran `overcommit --install` and all hooks pass
- [X] I tested my changes thoroughly
- [X] I updated documentation if needed
- [X] I didn't modify auto-generated files manually (`models.json`,
`aliases.json`)

## API changes

- [ ] Breaking change
- [ ] New public methods/classes
- [ ] Changed method signatures
- [X] No API changes

## Related issues

<!-- Link issues: "Fixes #123" or "Related to #123" -->
Related to #226

---------

Co-authored-by: Carmine Paolino <carmine@paolino.me>

Author: Paul Shippy

docs/guides/rails.md

@@ -193,6 +193,29 @@ class ToolCall < ApplicationRecord
 end
 ```
 
+### Setup RubyLLM.chat yourself
+
+In some scenarios, you need to tap into the power and arguments of `RubyLLM.chat`. For example, if want to use model aliases with alternate providers. Here is a working example:
+
+```ruby
+ class Chat < ApplicationRecord
+    acts_as_chat
+
+    validates :model_id, presence: true
+    validates :provider, presence: true
+
+    after_initialize :set_chat
+
+    def set_chat
+      @chat = RubyLLM.chat(model: model_id, provider:)
+    end
+  end
+
+  # Then in your controller or background job:
+  Chat.new(model_id: 'alias', provider: 'provider_name')
+```
+
+
 ## Basic Usage
 
 Once your models are set up, the `acts_as_chat` helper delegates common `RubyLLM::Chat` methods to your `Chat` model:
@@ -510,4 +533,4 @@ end
 *   [Using Tools]({% link guides/tools.md %})
 *   [Streaming Responses]({% link guides/streaming.md %})
 *   [Working with Models]({% link guides/models.md %})
-*   [Error Handling]({% link guides/error-handling.md %})
+*   [Error Handling]({% link guides/error-handling.md %})