Documentation about WPGraphQL and Querying data with Gatsby's GraphQL API

[jasonbahl]

This PR accomplishes the following:

• add .idea to the .gitignore to ignore hidden files from PHPStorm (and other JetBrains IDEs)
• remove using data link from main README (it's under tutorials)
• add link to GraphQL Schema Merging and Data Fetching doc in the features/index.md
• add Querying Data link in the tutorials.md
• Added contents to graphql-wordpress-and-gatsby.md
• Added contents to querying-data.md tutorial
• Added many images for the associated content