- Bumps @version in mix.exs from 9.0.0 to 9.0.1.
- Adds the CHANGELOG 9.0.1 section covering the docs/hexdocs
  overhaul (ADRs unpublished from hexdocs, link fixes including
  the readme.html collision, contributing guide published, badge
  row, zero mix docs warnings).
- Leaves the README install pin at ~> 9.0: a patch release does
  not move it.

Refs: px-7ok