Summary:
- set the project version to 0.3.1 and add the 0.3.1 changelog
  notes
- add a forced docs alias and tighten simulator, domain, and DC
  moduledocs for the current release state
- keep moved hardware scripts out of the default test sweep and
  rename the UDP transport script so mix test stays clean

Rationale:
- ship the current main changes in the 0.3.x line so
  kino_ethercat can consume them without a dependency range bump
- make the release validation path trustworthy by removing hidden
  ExDoc warnings and accidental hardware script execution during
  mix test

Validation:
- mix test
- mix docs.fresh
- mix hex.build

Co-authored-by: Codex <codex@openai.com>