[docs][guides] New guide on working with Etsy

[AishaBlake]

Summary

As part of the ongoing effort to improve on the top 25 learning workflows identified in #13708, we'd like to add a Reference Guide on working with Etsy in Gatsby. This will be one of several guides added as part of the e-commerce workflow.

Some recommended topics to cover:

• gatsby-source-etsy
• Etsy API
• getting product information
• cart functionality
• Opportunity for a new plugin/starter?

Motivation

The ability to build e-commerce sites with Gatsby gives developers (and business owners) a lot of flexibility. We'd like to provide a starting point for folks using some of the most popular e-commerce tools.

Steps to resolve this issue

Draft the doc

• Write the doc, following the format listed in these resources:
  • Overview on contributing to documentation
  • Docs Templates
  • Example of a guide to imitate
  • Example of a similar article
• Add the article to the docs sidebar under the "Building an E-commerce Site" section.

Open a pull request

• Open a pull request with your work including the words "closes #[this issue's number]" in the pull request description