fix(baseline): hide on /MDN/ pages

[caugner]

Summary

Fixes #11328.

Problem

The Baseline banner was also shown on the Compatibility tables and the browser compatibility data repository (BCD) page, which is part of the MDN Writing Guidelines.

Solution

Hide the Baseline banner in the MDN meta docs.

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Screenshots

Before

After

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How did you test this change?

Ran yarn dev and checked these pages locally:

• http://localhost:3000/en-US/docs/MDN/Writing_guidelines/Page_structures/Compatibility_tables
• http://localhost:3000/en-US/docs/Web/CSS/grid