Update notebook metadata for docs

[sdgilley]

Description

Adding cell metadata to use for docs. Also switched model as the Mistral-Large-2407 is no longer available.

Checklist

• I have read the contribution guidelines
• I have coordinated with the docs team (mldocs@microsoft.com) if this PR deletes files or changes any file names or file extensions.
• This notebook or file is added to the CODEOWNERS file, pointing to the author or the author's team.

[Copilot]

Pull Request Overview

This PR updates notebook metadata for enhanced documentation and replaces the outdated model identifier with the new one. Key changes include:

• Adding descriptive "name" metadata to code cells in two notebooks.
• Updating the model identifier from "mistral-large-2407" to "mistral-medium-2505" throughout both notebooks.
• Updating the CODEOWNERS file to reflect the appropriate documentation team ownership.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
scenarios/langchain/getting-started-with-langchain-embeddings.ipynb	Added cell metadata "name" for better clarity and consistency.
scenarios/langchain/getting-started-with-langchain-chat-models.ipynb	Added cell metadata "name" and updated the model identifier in code cells to the new deployment model.
.github/CODEOWNERS	Updated ownership rules to include new files referenced in documentation.