Documentation-only release for the hexdocs overhaul: ADRs
unpublished, README badge row added, install snippet corrected to
~> 0.2, and mix docs now completes with zero warnings.

Bumps the version in mix.exs and authors the 0.2.1 CHANGELOG
section directly - the docs work wrote no changelog.d fragments,
per the fragment convention's docs exclusion.

Refs: sob-8hh