Bumps the package version 2.1.0 -> 2.1.1 in mix.exs and authors the
CHANGELOG section for the docs/hexdocs overhaul directly (that work
deliberately wrote no changelog.d fragments): ADRs unpublished from
hexdocs, guide and README links converted to resolve on both hexdocs
and GitHub, the README rebuilt, and all 69 lib docstring reference
warnings fixed so mix docs builds with zero warnings.

The README install snippet ("~> 2.0") already covers the 2.1 series,
so it is unchanged. Tagging and hex publish remain operator actions.

Refs: st-ty7v