Add mapping example for Markdown files (#7650)
Add mapping example for Markdown files (#7650)

* Add links to config options in 'gatsby config' doc

* Add info about mapping between Markdown files

ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,0x80/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,0x80/gatsby,gatsbyjs/gatsby