docs: Add the firestore_setup_client_create_with_project_id region tag

[Mariatta]

The Firestore quickstart_new_instance sample can be used to demonstrate setting up client using a project id.

With this region tag, we can include it into this doc: https://cloud.google.com/firestore/docs/samples/firestore-setup-client-create-with-project-id

Description

Fixes internal issue 192501359

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Please merge this PR for me once it is approved.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• firestore/cloud-client/snippets.py:24, tag firestore_setup_client_create_with_project_id

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