docs: Make group for module documentation
docs: Make group for module documentation

This change extracts module documentation deps into their own GN group
usable by downstream projects.

Change-Id: I49eb211cbbf13bb105a739729e64cebf73722d26
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/17623
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Commit-Queue: Alexei Frolov <a1b001416a8cf9aaea32a3411f8f3c0793ebc0b5@google.com>

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