[SPARK-3946] gitignore in /python includes wrong directory

Modified to ignore not the docs/ directory, but only the docs/_build/ which is the output directory of sphinx build.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #2796 from tsudukim/feature/SPARK-3946 and squashes the following commits:

2bea6a9 [Masayoshi TSUZUKI] [SPARK-3946] gitignore in /python includes wrong directory

python/.gitignore

 *.pyc
-docs/
+docs/_build/
 pyspark.egg-info
 build/
 dist/