This PR adds docs for the Rack library, which is arguably the most important Ruby library when it comes to web development. Anyone who regularly uses Rails or one of the other Ruby web frameworks will likely need to consult the Rack docs at some point.

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If you’re adding a new scraper, please ensure that you have:

• Tested the scraper on a local copy of DevDocs
• Ensured that the docs are styled similarly to other docs on DevDocs

• Added these files to the public/icons/your_scraper_name/ directory:
  • 16.png: a 16×16 pixel icon for the doc
  • 16@2x.png: a 32×32 pixel icon for the doc
  • SOURCE: A text file containing the URL to the page the image can be found on or the URL of the original image itself