Add X-Tenant-Id header info in the API key section

[romain-priour-lc]

Overview

Customers didn't know to include X-tenant-Id header. Doesn't hurt to mention it in the section where we explain what each API key is.

Type of change

Type:
Update existing documentation

To automatically close an issue when this PR is merged, use closing keywords:

• Closes INF-868

• Linear issue: https://linear.app/langchain/issue/INF-868/add-to-docs-that-you-need-to-add-x-tenant-id-in-the-header-with

Checklist

• I have read the contributing guidelines
• I have tested my changes locally using docs dev
• [] All code examples have been tested and work correctly
• [] I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed
• I have gotten approval from the relevant reviewers
• (Internal team members only / optional) I have created a preview deployment using the Create Preview Branch workflow

Additional notes

[github-actions]

Preview ID generated: preview-romain-1759204940-38c240f

[github-actions]

Preview ID generated: preview-romain-1759205276-b5d40a5

[katmayb]

Non-blocking suggestion, but lgtm!

[github-actions]

Preview ID generated: preview-romain-1759367400-8e495c7