message: docs: update the extensions page
diff: Extending Envoy for custom use cases
  ====================================
  -The Envoy architecture makes it fairly easily extensible via both :ref:`network filters
  -<arch_overview_network_filters>` and :ref:`HTTP filters <arch_overview_http_filters>`.
  +The Envoy architecture makes it fairly easily extensible via a variety of differerent extension
  +types including:
  +
  +* :ref:`Access loggers <arch_overview_access_logs>`
  +* :ref:`Clusters <arch_overview_service_discovery>`
  +* :ref:`Listener filters <arch_overview_listener_filters>`
  +* :ref:`Network filters <arch_overview_network_filters>`
  +* :ref:`HTTP filters <arch_overview_http_filters>`
  +* :ref:`gRPC credential providers <arch_overview_grpc>`
  +* :ref:`Health checkers <arch_overview_health_checking>`
  +* :ref:`Resource monitors <arch_overview_overload_manager>`
  +* :ref:`Retry implementations <arch_overview_http_routing_retry>`
  +* :ref:`Stat sinks <arch_overview_statistics>`
  +* :ref:`Tracers <arch_overview_tracing>`
  +* Transport sockets
  +
  +As of this writing there is no high level extension developer documentation. The
  +:repo:`existing extensions <source/extensions>` are a good way to learn what is possible.
  An example of how to add a network filter and structure the repository and build dependencies can
  be found at `envoy-filter-example <https://github.com/envoyproxy/envoy-filter-example>`_.
  