[release-1.9] Make extension doc generates snake case.

[istio-testing]

This is an automated cherry-pick of #3191

[lambdai]

/retest

[lambdai]

[2021-02-23 05:23:38.174][28577][info][main] [external/envoy/source/server/server.cc:790] shutting down server instance
[2021-02-23 05:23:38.174][28577][info][main] [external/envoy/source/server/server.cc:842] Notifying 0 callback(s) with completion.
[2021-02-23 05:23:38.176][28577][info][main] [external/envoy/source/server/server.cc:736] main dispatch loop exited
[2021-02-23 05:23:38.202][28577][warning][config] [bazel-out/k8-fastbuild/bin/external/envoy/source/common/config/_virtual_includes/grpc_stream_lib/common/config/grpc_stream.h:101] StreamAggregatedResources gRPC config stream closed: 13, 
[2021-02-23 05:23:38.236][28577][info][main] [external/envoy/source/server/server.cc:782] exiting
[2021-02-23 05:23:38.282][28577][critical][assert] [external/envoy/source/common/stats/symbol_table_impl.cc:194] assert failure: numSymbols() == 0.

We should cherry pick envoyproxy/envoy#14954 to fix this

[bianpengyuan]

/retest