feat: llms.txt for Ionic / Capacitor

### Prerequisites

- [x] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/docs/CONTRIBUTING.md#creating-an-issue).
- [x] I agree to follow the [Code of Conduct](https://ionicframework.com/code-of-conduct).
- [x] I have searched for [existing issues](https://github.com/ionic-team/ionic-framework/issues) that already include this feature request, without success.

### Describe the Feature Request

Dear Ionic Team,

I noticed that projects like Angular (https://angular.dev/llms.txt) and CapAwesome (https://capawesome.io/llms.txt) provide an llms.txt file to help LLMs better understand and reference their documentation.

Is there any plan to add a similar /llms.txt endpoint for Ionic and/or Capacitor?
(I could generated a file, but an official version would be more useful)

Thanks!

### Describe the Use Case

It would help AI tools provide more accurate and structured answers based on the official docs.

### Describe Preferred Solution

_No response_

### Describe Alternatives

_No response_

### Related Code

_No response_

### Additional Information

_No response_

Start by reviewing how the Ionic and Capacitor documentation is published, then compare the linked Angular and CapAwesome /llms.txt examples. Decide what official content the endpoint should cover and how it should be exposed; done means an agreed, maintained /llms.txt endpoint for the relevant documentation.

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