Add a README terminal demo GIF

[JohnnyWilson16]

Summary

Add a short terminal GIF or screenshot to the README showing fd.clean(df) and
report.summary() on the quickstart DataFrame.

Acceptance Criteria

• The asset is committed under a stable docs or assets path.
• The README first screen shows the demo without pushing install instructions below the fold.
• The PR includes alt text and a before/after note.

From issue-backlog.md §1.

[WilliamK112]

I implemented this: added a terminal demo GIF under docs/assets/demo and updated the README front section with alt text + before/after note. Opened PR #15 to close #11.