Adding the list of APIs we won't support in Koalas to docs.

[itholic]

Inspired by #1261 , we'd better to have the list of APIs which are not planned to implement to the official docs.

[itholic]

I roughly listed-up once and added them to FAQ section.

If there is a missing elements or any idea for improving this documentation, just let me know so that i can add to list while i'm here.

[codecov-io]

Codecov Report

Merging #1270 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1270   +/-   ##
=======================================
  Coverage   95.16%   95.16%           
=======================================
  Files          35       35           
  Lines        7151     7151           
=======================================
  Hits         6805     6805           
  Misses        346      346

Impacted Files	Coverage Δ
databricks/koalas/usage_logging/__init__.py	97.29% <0%> (ø)	:arrow_up:
databricks/koalas/usage_logging/usage_logger.py	100% <0%> (ø)	:arrow_up:
databricks/koalas/__init__.py	85.1% <0%> (ø)	:arrow_up:
databricks/conftest.py	96.22% <0%> (ø)	:arrow_up:

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