Improve README: banner, absolute links, standard sections

[jfrosorio]

The README on main needs a quality pass to match what users expect from an established package:

• Add the package banner image at the top (art/banner.png), referenced by absolute URL so it renders when the README is republished outside GitHub.
• Replace all relative links (e.g. (UPGRADE.md)) with absolute GitHub URLs — relative links 404 when the README is pulled into codetech.pt.
• Add a Packagist total-downloads badge.
• Add a Changelog section linking to GitHub releases.
• Add a short support/star note.
• The v1 and v2 branch READMEs have the same relative upgrade-guide link (plus LICENSE links to a nonexistent master branch) and need the equivalent fix.

Execution is tracked in the sub-issues below, one per PR. This issue stays open until all sub-issues are closed.

[jfrosorio]

All three sub-issues (#27, #28, #29) are closed — README revamp landed on main via #20 and the legacy-branch link fixes via #21/#22.