ARROW-346: Use conda environment to build API docs

As we cannot currently build pyarrow on readthedocs, we have to resort
to building the API docs for the latest version of pyarrow on
conda-forge. All other documentation will though be pulled directly from
the source code.

Change-Id: I97618817d0f6b48ecb9054b8c898097697b182a9

.readthedocs.yml

@@ -0,0 +1,2 @@
+conda:
+    file: python/doc/environment.yml

python/doc/conf.py

@@ -42,7 +42,12 @@ cmd_line_template = "sphinx-apidoc -f -e -o {outputdir} {moduledir}"
 cmd_line = cmd_line_template.format(outputdir=output_dir, moduledir=module_dir)
 apidoc.main(cmd_line.split(" "))
 
-sys.path.insert(0, os.path.abspath('..'))
+on_rtd = os.environ.get('READTHEDOCS') == 'True'
+
+if not on_rtd:
+    # Hack: On RTD we use the pyarrow package from conda-forge as we cannot
+    # build pyarrow there.
+    sys.path.insert(0, os.path.abspath('..'))
 
 # -- General configuration ------------------------------------------------
 

python/doc/environment.yml

@@ -0,0 +1,8 @@
+channels:
+- defaults
+- conda-forge
+dependencies:
+- arrow-cpp
+- parquet-cpp
+- pyarrow
+- numpydoc