[oss-timeseries] migrate internal design doc on routing (#4167)
[oss-timeseries] migrate internal design doc on routing (#4167)

Adds a design doc on frontend routing, originally created internally.

Test plan:
Ran a test sync and saw that this creates an empty sync
cl/331809580
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard