docs(media): add an example for using a custom JSON encoder

[maxking]

Summary of Changes

Adds an example on how to override the default json encoder used by Falcon.

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[codecov[bot]]

Codecov Report

Merging #2035 (d5418c0) into master (43a8e80) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #2035   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           63        63           
  Lines         6707      6707           
  Branches      1238      1238           
=========================================
  Hits          6707      6707           

Impacted Files	Coverage Δ
falcon/media/json.py	100.00% <ø> (ø)

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[maxking]

Working on adding a reference back from FAQ, Sphinx doesn't seem to like sections inside the docstrings and I am trying to see how to create a reference without a section in a paragraph. Will update the PR as soon as I can figure that out. :D

[maxking]

Thanks @vytas7, updated the PR.