Check if metadata is present in build script

[kkerti]

On each page in this docusaurus 2.0 project, the meta data between the --- --- semantics are used for page title, description, tags and other details.

The documentation writers should pay attention the these tags, but sometimes they can be forgotten.

• Create a a script which checks the created pages, if they have the following meta data on the top: description, title, pageImg, slug
• Throw error during build if any of the pages don't have the above 4 fields
• Make the error editor friendly, show which page caused error: has missing field tags!