Drop htmlzip download format from readthedocs, leaving only pdf

[dsherry]

We're still seeing RTD timeouts on @angela97lin 's feature branch for #346 , so dropping down to PDF as our only download artifact.

[codecov]

Codecov Report

Merging #600 into master will increase coverage by 14.48%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #600       +/-   ##
===========================================
+ Coverage   84.39%   98.87%   +14.48%     
===========================================
  Files         118      118               
  Lines        4453     4453               
===========================================
+ Hits         3758     4403      +645     
+ Misses        695       50      -645     

Impacted Files	Coverage Δ
evalml/pipelines/pipeline_base.py	98.53% <0.00%> (+0.48%)	⬆️
evalml/tests/component_tests/test_components.py	100.00% <0.00%> (+0.62%)	⬆️
evalml/automl/auto_base.py	96.19% <0.00%> (+0.69%)	⬆️
evalml/tests/pipeline_tests/test_pipelines.py	99.63% <0.00%> (+2.94%)	⬆️
evalml/tests/component_tests/test_utils.py	96.42% <0.00%> (+3.57%)	⬆️
evalml/tests/utils_tests/test_dependencies.py	100.00% <0.00%> (+6.25%)	⬆️
evalml/pipelines/classification/xgboost.py	100.00% <0.00%> (+12.50%)	⬆️
...nents/estimators/classifiers/xgboost_classifier.py	100.00% <0.00%> (+13.63%)	⬆️
evalml/pipelines/regression/catboost.py	100.00% <0.00%> (+14.28%)	⬆️
evalml/pipelines/classification/catboost.py	100.00% <0.00%> (+14.28%)	⬆️
... and 15 more

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[jeremyliweishih]

LGTM. Do we actually need any of these artifacts? If deleting this doesn't work either it would lead me to think that it might be another issue.

[dsherry]

Looks like this worked on our docs branch! @angela97lin you wanna approve this so we can get it in and update the feature branch?

[angela97lin]

YAY for build succeeding! :'D

[dsherry]

@jeremyliweishih yeah it seems sphinx is taking over double the time to build on the improved_docs feature branch. I don't understand why. Our theory is that it's because that branch adds 15-20 classes to the API ref. But that doesn't really explain why. We can fine tune that once we've got it into master.

[dsherry]

And yeah I think it'd important to keep some downloadable format. PDF is great.