add CBOR example

[grisumbras]

Fix #366

[cppalliance-bot]

An automated preview of the documentation is available at https://1066.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1066/pullrequest.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.70%. Comparing base (57d42f0) to head (159a8f3).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1066   +/-   ##
========================================
  Coverage    93.70%   93.70%           
========================================
  Files           91       91           
  Lines         9139     9139           
========================================
  Hits          8564     8564           
  Misses         575      575           

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