docs: restructure README with a quick-start section

[Chemaclass]

🤔 Background

The README sent every reader to bashunit.com for everything and leaned on marketing prose ("most modern", "very active community"), so it delivered no value on the page itself.

💡 Changes

• Add a Quick start (install one-liner, a minimal test, the run command) so users can get going without leaving the page.
• Trim the tagline and description to remove marketing fluff.
• Drop the standalone Requirements section (folding "Bash 3.0+" into the intro) and tighten Contribute.

Stacked on #732 (Publish Docs badge removal). Retarget to main once that merges.