feat(gatsby-plugin-netlify-cms): add custom favicon to cms section

[chrisjpatty]

Description

This pull request adds the ability to define an optional favicon field in the plugin, gatsby-plugin-netlify-cms. It leverages the favicon option defined by the HTML Webpack Plugin. I've also updated the relevant documentation page describing how to add the property.

This is my first PR to this repo, let me know if there's anything additional you need to review this PR, thanks!

[wardpeet]

Looks good and works 👍

i've tested it with https://github.com/netlify-templates/gatsby-starter-netlify-cms

[gatsbot]

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[wardpeet]

Published in gatsby-plugin-netlify-cms@4.1.5