message: Add conceptual documentation for NetEq.
  Many things are omitted in this doc and it can definitely be improved,
  but I hope it captures the most important parts.
  Cr-Commit-Position: refs/heads/master@{#33724}
diff: * [SCTP](/pc/g3doc/sctp_transport.md)
  * Congestion control and bandwidth estimation
  * Audio
  + * [NetEq](/modules/audio_coding/neteq/g3doc/index.md)
  * AudioEngine
  * [ADM](/modules/audio_device/g3doc/audio_device_module.md)
  * [Audio Coding](/modules/audio_coding/g3doc/index.md)
  