Add favicon to doc

[harupy]

This PR adds a favicon to the doc.

[codecov-io]

Codecov Report

Merging #1189 into master will decrease coverage by 2.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1189      +/-   ##
==========================================
- Coverage   95.24%   92.72%   -2.52%     
==========================================
  Files          35       35              
  Lines        7105     7105              
==========================================
- Hits         6767     6588     -179     
- Misses        338      517     +179

Impacted Files	Coverage Δ
databricks/koalas/usage_logging/__init__.py	24.32% <0%> (-72.98%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50% <0%> (-50%)	⬇️
databricks/koalas/__init__.py	78.72% <0%> (-6.39%)	⬇️
databricks/conftest.py	92% <0%> (-6%)	⬇️
databricks/koalas/frame.py	93.74% <0%> (-3.27%)	⬇️
databricks/koalas/groupby.py	89.6% <0%> (-2%)	⬇️
databricks/koalas/namespace.py	86.06% <0%> (-1.52%)	⬇️
databricks/koalas/plot.py	93.33% <0%> (-0.96%)	⬇️
databricks/koalas/testing/utils.py	77.77% <0%> (-0.75%)	⬇️
databricks/koalas/utils.py	95.09% <0%> (-0.62%)	⬇️
... and 1 more

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[ueshin]

Thanks! merging.

[harupy]

It seems to be working fine.