pw_protobuf: Restructure docs
pw_protobuf: Restructure docs

To make the documentation easier to follow, especially once new support
is documented, restructure so that code generation is a top-level item
separate to encoding and decoding.

Change-Id: I5ee30e16f0a0c257954020f5eca6c66b8eeb8c46
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/89422
Pigweed-Auto-Submit: Scott James Remnant <6b88cbe64f92c5d64ffbf57d7b3d2efc0467fe34@google.com>
Reviewed-by: Alexei Frolov <a1b001416a8cf9aaea32a3411f8f3c0793ebc0b5@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed