ARROW-2014: [Python] Document read_pandas method in pyarrow.parquet
ARROW-2014: [Python] Document read_pandas method in pyarrow.parquet

Added read_pandas to the parquet documentation. Added a custom index to show that it is maintained when providing a subset of columns to read_pandas.

Author: Alex <alex@unexpectedeof.net>

Closes #1820 from AlexHagerman/arrow-2014 and squashes the following commits:

44f4412 <Alex> Added  to parquet documentation per ticket 2014

xhochy/arrow,pcmoritz/arrow,laurentgo/arrow,itaiin/arrow,kou/arrow,wesm/arrow,apache/arrow,wesm/arrow,renesugar/arrow,icexelloss/arrow,renesugar/arrow,xhochy/arrow,xhochy/arrow,tebeka/arrow,itaiin/arrow,icexelloss/arrow,majetideepak/arrow,wagavulin/arrow,pcmoritz/arrow,wagavulin/arrow,majetideepak/arrow,itaiin/arrow,kou/arrow,icexelloss/arrow,icexelloss/arrow,tebeka/arrow,majetideepak/arrow,laurentgo/arrow,wesm/arrow,itaiin/arrow,cpcloud/arrow,renesugar/arrow,majetideepak/arrow,pcmoritz/arrow,tebeka/arrow,itaiin/arrow,apache/arrow,wesm/arrow,itaiin/arrow,itaiin/arrow,icexelloss/arrow,cpcloud/arrow,xhochy/arrow,pcmoritz/arrow,majetideepak/arrow,tebeka/arrow,majetideepak/arrow,pcmoritz/arrow,kou/arrow,laurentgo/arrow,renesugar/arrow,icexelloss/arrow,renesugar/arrow,pcmoritz/arrow,wagavulin/arrow,apache/arrow,apache/arrow,laurentgo/arrow,majetideepak/arrow,kou/arrow,tebeka/arrow,pcmoritz/arrow,xhochy/arrow,cpcloud/arrow,wagavulin/arrow,kou/arrow,xhochy/arrow,kou/arrow,wagavulin/arrow,wesm/arrow,wagavulin/arrow,majetideepak/arrow,kou/arrow,apache/arrow,apache/arrow,icexelloss/arrow,pcmoritz/arrow,cpcloud/arrow,itaiin/arrow,renesugar/arrow,wagavulin/arrow,xhochy/arrow,wagavulin/arrow,kou/arrow,laurentgo/arrow,wesm/arrow,pcmoritz/arrow,itaiin/arrow,cpcloud/arrow,kou/arrow,xhochy/arrow,itaiin/arrow,laurentgo/arrow,renesugar/arrow,majetideepak/arrow,wagavulin/arrow,icexelloss/arrow,apache/arrow,apache/arrow,wesm/arrow,wesm/arrow,pcmoritz/arrow,itaiin/arrow,apache/arrow,pcmoritz/arrow,tebeka/arrow,icexelloss/arrow,majetideepak/arrow,icexelloss/arrow,wesm/arrow,cpcloud/arrow,cpcloud/arrow,wesm/arrow,cpcloud/arrow,laurentgo/arrow,kou/arrow,itaiin/arrow,renesugar/arrow,kou/arrow,wagavulin/arrow,renesugar/arrow,xhochy/arrow,xhochy/arrow,apache/arrow,cpcloud/arrow,pcmoritz/arrow,wesm/arrow,icexelloss/arrow,wagavulin/arrow,kou/arrow,xhochy/arrow,icexelloss/arrow,renesugar/arrow,majetideepak/arrow,pcmoritz/arrow,apache/arrow,tebeka/arrow,icexelloss/arrow,renesugar/arrow,cpcloud/arrow,pcmoritz/arrow,cpcloud/arrow,icexelloss/arrow,xhochy/arrow,tebeka/arrow,majetideepak/arrow,laurentgo/arrow,laurentgo/arrow,wagavulin/arrow,cpcloud/arrow,renesugar/arrow,majetideepak/arrow,itaiin/arrow,xhochy/arrow,itaiin/arrow,cpcloud/arrow,majetideepak/arrow,cpcloud/arrow,tebeka/arrow,itaiin/arrow,laurentgo/arrow,xhochy/arrow,renesugar/arrow,wesm/arrow,wesm/arrow,apache/arrow,laurentgo/arrow,laurentgo/arrow,kou/arrow,xhochy/arrow,laurentgo/arrow,apache/arrow,wesm/arrow,kou/arrow,renesugar/arrow,renesugar/arrow,pcmoritz/arrow,tebeka/arrow,majetideepak/arrow,laurentgo/arrow,apache/arrow,laurentgo/arrow