[Docs] Fix "Adding Markdown Pages" url (#3021)
[Docs] Fix "Adding Markdown Pages" url (#3021)

In *Guides* section *Adding Markdown Pages* url is https://www.gatsbyjs.org/adding-markdown-pages/, which is  currently 404. it should be https://www.gatsbyjs.org/docs/adding-markdown-pages/
ChristopherBiscardi/gatsby,fabrictech/gatsby,fabrictech/gatsby,0x80/gatsby,fabrictech/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,danielfarrell/gatsby,Khaledgarbaya/gatsby,danielfarrell/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,fk/gatsby,mingaldrichgan/gatsby,fk/gatsby,danielfarrell/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,0x80/gatsby,Khaledgarbaya/gatsby,Khaledgarbaya/gatsby,0x80/gatsby,gatsbyjs/gatsby