[Metrics] [Docs] Add section about device placement

[SkafteNicki]

What does this PR do?

Fixes #4909
Adds a section to metrics docs about device placement

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[codecov]

Codecov Report

Merging #5280 (d9c8134) into master (0c7c9e8) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5280   +/-   ##
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  Coverage      93%     93%           
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  Files         134     134           
  Lines        9950    9950           
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  Hits         9261    9261           
  Misses        689     689           

[Shreeyak]

Beautiful. Thanks for adding these details. I narrowly avoided using a native dict through luck.