Rewrite the README around screenshots of the running editor

Adds a feature tour carried by eighteen captures of the application itself: a
real run with its output, ruff diagnostics, a failing pytest with its traceback,
the git client showing a diff, the terminal and the embedded browser.

Also corrects the run-configuration plugin example, which passed keyword
arguments and a 'command'/'suffix' pair that register_plugin_run_config has
never accepted; it takes one dict keyed by suffixes, compiler and args.

Author: JE-Chen