doc generation: add sha256 hashes to requirements.txt (#1329)
doc generation: add sha256 hashes to requirements.txt (#1329)

Description: Allows safer installation of python dependencies.
Risk Level: Low
Testing: CI

Signed-off-by: Mike Schore <270e364e708f2af319f244c09ceba52a03dbd980@gmail.com>
envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile