Split project/crate README

[robo9k]

Partially implements #106 (no website).
Splits the rust-magic project README from the magic crate README.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (35fdbce) 40.88% compared to head (5f9374b) 40.88%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #124   +/-   ##
=======================================
  Coverage   40.88%   40.88%           
=======================================
  Files           2        2           
  Lines         587      587           
  Branches      587      587           
=======================================
  Hits          240      240           
  Misses        346      346           
  Partials        1        1           

Files	Coverage Δ
src/lib.rs	38.15% <ø> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.