[docs][guides] New guide on porting from Jekyll to Gatsby

[marcysutton]

As a Jekyll user interested in migrating my site to Gatsby, I should find a section in the docs explaining how to do it. This new doc would live in the Porting to Gatsby section alongside the pages on Create React App and HTML.

@laurieontech's blog post series on porting from Jekyll to Gatsby could be a great starting point: https://tenmilesquare.com/rewriting-a-static-website-using-gatsby-and-graphql/

(Part 2 - https://tenmilesquare.com/rewriting-a-static-website-using-gatsby-and-graphql-part-2/)
(Part 3 - https://tenmilesquare.com/rewriting-a-static-website-using-gatsby-and-graphql-part-3/)

For a docs guide PR to be accepted, it needs to follow the Reference Guide format and Gatsby Style Guide:

• https://www.gatsbyjs.org/contributing/docs-templates/
• https://www.gatsbyjs.org/contributing/gatsby-style-guide/