docs: add section about site-level landing page to products documentation

[devin-ai-integration]

Summary

Adds a new section to the products documentation page explaining how to configure a site-level landing page that is not linked to any product. This helps users understand that they can have an introductory page at the root of their documentation site before users select a product.

The new section includes:

• Brief explanation of the use case
• Code example showing the landing-page configuration in docs.yml
• Link to the existing landing page configuration reference for detailed options

Review & Testing Checklist for Human

• Verify the link /learn/docs/configuration/site-level-settings#landing-page-configuration resolves correctly in the preview
• Confirm the section placement makes sense (between "Add products to your docs" and "Add versioning to your products")
• Check that the code example is accurate and follows the correct YAML syntax for the landing-page configuration

Notes

• Vale style checks could not be run locally (not installed), but CI should catch any style issues
• No changelog entry was added since this documents an existing feature rather than a new one - please confirm this is the correct approach

Link to Devin run: https://app.devin.ai/sessions/dfe5b1be3ece42158137b0c6e11e4a73
Requested by: Fern Support ([email protected])

[devin-ai-integration]

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[github-actions]

🚫 [vale] _{reported by reviewdog 🐶}
[Microsoft.Contractions] Use 'isn't' instead of 'is not'.

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