Add Crates.io recent downloads badge (fixes #4186), run [Crates]

[17cupsofcoffee]

Summary of the changes:

• Added a 'recent downloads' badge for Crates.io.
  • I exposed it at /crates/dr/:crate (r for recent), as this seemed consistent with the existing routes in that namespace.
  • The data for recent downloads is only available for the crate as a whole, not individual versions, so I made the service bail out early with an error if you try to do that, to avoid unneccecary calls to Crates.io.
• Cleaned up some of the existing code.
• Updated the examples.
  • I left /d/:crate/:version out of the examples, because it does the same thing as /dt/:crate/:version and the latter makes more sense to me. The URL still works, though.
  • The URLs in the examples now include the relevant variants instead of :variant - this seems to play nicer with the modal editor.

Hopefully this is okay, first time contributing :)

[shields-ci]

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[calebcartwright]

Thanks so much!

[17cupsofcoffee]

No problem, thank you for the helpful review comments 😄

[shields-deployment]

This pull request was merged to master branch. This change is now waiting for deployment, which will usually happen within a few days. Stay tuned by joining our #ops channel on Discord!

After deployment, changes are copied to gh-pages branch: