Bring in elixxir/docs (cMixx Design Specs)

website/docs/cmixx/design-specs/README.md

website/docs/cmixx/design-specs/architecture.md

website/docs/cmixx/design-specs/cmixx.md

website/docs/cmixx/design-specs/dm.md

website/docs/cmixx/design-specs/end_to_end.md

website/docs/cmixx/design-specs/gateway_gossip.md

website/docs/cmixx/design-specs/gateway_mixing.md

website/docs/cmixx/design-specs/message_pickup.md

website/docs/cmixx/design-specs/multicast_channel.md

website/docs/cmixx/design-specs/pki.md

website/docs/cmixx/design-specs/threat_model.md

website/docs/cmixx/design-specs/wire.md

website/docs/intro.md

website/static/img/auth_protocol.png

website/static/img/channel_traffic_analysis.png

website/static/img/client_proxy_gw_node_request_key.png

website/static/img/gateway_mixing.png

website/static/img/message_send.png

website/static/img/rekey_fsm.png

website/static/img/shared_secret_ladder.png