[docs][guides] New Reference Guide on working with WooCommerce

[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 WooCommerce in Gatsby. This will be one of several guides added as part of the e-commerce workflow.

Some recommended topics to cover:

• note on headless WordPress
• any overlap with gatsby-source-wordpress
• existing plugins
• getting product information
• cart functionality

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

[TylerBarnes]

Great idea! Heads up that there will be a new major version of gatsby-source-wordpress coming out in the new year and that will likely change what the best way to use Woo with Gatsby is. https://github.com/gatsbyjs/gatsby/issues/19292

Most likely once we have the new source plugin, using the wp-graphql-woocommerce plugin will be the way to go. We've been talking about making a Woo theme for the new source plugin as well!