Add a short how-to article for removing the 8px margin in html body

[dianacheung]

Preliminary Checks

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• This issue is not a question, feature request, RFC, or anything other than a bug report. Please post those things in GitHub Discussions: https://github.com/gatsbyjs/gatsby/discussions

Summary

I changed my background color to black and noticed a 8px margin in the html body. I did a google search and this is a common issue users run into. The quick fix should be added in a how-to article in gatsbyjs docs. I tried the fix listed at #28821 which removed the 8px margin.

Steps to Resolve this Issue

1. Create a styles.css file at the root of your project
2. Create a gatsby-browser.js at the root of your project and import the styles.css file