Deprecate document conversion

[ehdsouza]

The deprecation message is as follows:

(MY_ENV)  python examples/document_conversion_v1.py
examples/document_conversion_v1.py:17: DeprecationWarning: convert_document is a deprecated function. Since Document Conversion Service was retired in October 2017, we have continued to improve document conversion capabilities within Watson Discovery. If you are a Document Conversion user, get started with Discovery today. Refer to the migration guide: https://console.bluemix.net/docs/services/discovery/migrate-dcs-rr.html

[codecov-io]

Codecov Report

Merging #354 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #354      +/-   ##
==========================================
+ Coverage   38.29%   38.39%   +0.09%     
==========================================
  Files          24       25       +1     
  Lines        8706     8720      +14     
==========================================
+ Hits         3334     3348      +14     
  Misses       5372     5372

Impacted Files	Coverage Δ
watson_developer_cloud/utils.py	100% <100%> (ø)
watson_developer_cloud/document_conversion_v1.py	94.59% <100%> (+0.84%)	⬆️

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