bazel: Add sphinxdocs as dev_dependency

Sphinx features were previously bundled as part of rules_python. They
are now published in their own Bazel module. Note that the code still
lives within rules_python repo. For now we can only take a dependency
on the module. After we make some infra changes we should be able to
start actually using the module. Because we only use this module to
build our own docs we should be able to leave it as a dev_dependency.

Change-Id: I4136954540b6d615f08eeb99e5895bd8c7cb426c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/435634