Move the install steps above requirements and features so the first screen
answers what the widget is, what it looks like, and how to install it. Open with
a one-line pitch and an animated tour of the popup, and record the KDE Store
listing as an install path beside the release package, noting that the GitHub
release is the newest package and that the widget's own update check follows it.

The animation is built from the existing `readme-` popup smoke scenarios, so it
carries the same synthetic data and Breeze Dark palette as the still captures
and needs no new rendering path. The four frames are padded to one canvas with
the popup background color, which keeps the tab strip fixed while the selection
moves left to right.

Documentation impact: adds docs/codexbar-plasma-tour.gif, indexes it in
docs/README.md with its rebuild instructions, and packages it alongside the
existing README screenshots. Package grows from 600,600 to 689,850 bytes. No
behavior, default, or requirement changes, so CHANGELOG is untouched.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>