feat: OpenRouter intgeration release

[Amnah199]

Summary and motivation

Adding a sub-issue to track the requirements related to docs and release of a new OpenRouter integration.

Checklist

If the request is accepted, ensure the following checklist is complete before closing this issue.

Tasks

• [x] The code is documented with docstrings and was merged in the main branch
• [ ] Docs are published at https://docs.haystack.deepset.ai/
• [ ] There is a Github workflow running the tests for the integration nightly and at every PR
• [x] A new label named like integration:<your integration name> has been added to the list of labels for this repository
• [x] The labeler.yml file has been updated
• [ ] The package has been released on PyPI
• [ ] An integration tile has been added to https://github.com/deepset-ai/haystack-integrations
• [x] The integration has been listed in the Inventory section of this repo README
• [x] There is an example available to demonstrate the feature
• [ ] The feature was announced through social media