docs: Migrate to gp-sphinx theme and packages (#657)

## Summary

- Migrate docs to use `gp-sphinx` shared configuration platform via
  `merge_sphinx_config()`
- Pin gp-sphinx workspace packages to published `0.0.1a1`

## Test plan

- [x] Docs build passes in CI
- [x] Live docs publish to S3 from this branch

Author: tony