This PR is part of the issue #3492.
# Objective
- Clean up dead code in `bevy_core`.
- Add and update the `bevy_core` documentation to achieve a 100% documentation coverage.
- Add the #![warn(missing_docs)] lint to keep the documentation coverage for the future.
# Solution
- Remove unused `Bytes`, `FromBytes`, `Labels`, and `EntityLabels` types and associated systems.
- Made several types private that really only have use as internal types, mostly pertaining to fixed timestep execution.
- Add and update the bevy_core documentation.
- Add the #![warn(missing_docs)] lint.
# Open Questions
Should more of the internal states of `FixedTimestep` be public? Seems mostly to be an implementation detail unless someone really needs that fixed timestep state.