build: install dependencies needed to autogenerate reference docs (#1377)

Author: zimeg

scripts/generate_api_docs.sh

@@ -1,10 +1,15 @@
 #!/bin/bash
 # Generate API documents from the latest source code
 
-script_dir=`dirname $0`
-cd ${script_dir}/..
+set -e
+script_dir=$(dirname "$0")
+cd "${script_dir}/.."
 
+pip install -U pip
+pip install -U -r requirements/adapter.txt
+pip install -U -r requirements/async.txt
 pip install -U pdoc3
+pip install .
 rm -rf docs/reference
 
 pdoc slack_bolt --html -o docs/reference