New MongoDB source plugin!

Exciting to see the first source plugin for a database added!

Added

• Add gatsby-source-mongodb plugin to gatsby (#1570) @jorishermans
• [www] Refactor Homepage and Navigation & convert diagram to html/css (#1605) @fk
• Included example with WP-API-MENUS items (#1619) @sebastienfi
• added new site to showcase (#1616) @dvzrd
• Docs: add grommet starter (#1626) @alampros
• add a bunch of tests for various plugins (#1581) @DSchau

Fixed

• fix broken gatsby link definition for typescript 2.4.2 (#1628) @DominikGuzei
• test: fix failing test due to missing argyle image (#1636) @DSchau
• Revise part one of tutorial to use hello-world starter (#1630) @KyleAMathews
• Fix passing createNode as argument (#1629) @sebastienfi
• Don't mutate page context (#1537) @okcoker
• Updated instructions for Windows (#1621) @sebastienfi
• Tweak JSS links + add it to plugins page (#1615) @KyleAMathews