GPU docs

[mcabbott]

This re-writes the start of the GPU documentation page. It aims to use simpler examples, and stress that model |> cu just works, before talking about more exotic non-CUDA packages, and the automatic model |> gpu.

Rendered MD: https://github.com/FluxML/Flux.jl/blob/gpu_docs/docs/src/guide/gpu.md

Current docs: http://fluxml.ai/Flux.jl/stable/guide/gpu/

[github-actions]

Once the build has completed, you can preview any updated documentation at this URL: https://fluxml.ai/Flux.jl/previews/PR2510/ in ~20 minutes

In particular, this page: https://fluxml.ai/Flux.jl/previews/PR2510/guide/gpu/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.20%. Comparing base (32db5d4) to head (d9c03b9).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2510   +/-   ##
=======================================
  Coverage   33.20%   33.20%           
=======================================
  Files          31       31           
  Lines        1843     1843           
=======================================
  Hits          612      612           
  Misses       1231     1231           

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