[1.15.0] Update packaging to DSFR 1.15.1: new install method, etalab-2.0 license, version bump

Problem/Motivation

✨ Mise à jour de la licence du projet pour etalab-2.0 #1483
🎉 Ajout de conditions générales d'utilisation au build du projet #1346

Since 1.15 the compiled zip is not attached to GitHub releases anymore (the v1.15.x assets return 404). The DSFR team confirmed this is intentional: the new terms (Etalab 2.0) must be accepted at install, and the install procedure, including for CI, is documented in the upstream README.

Proposed resolution

• composer.json: bump the inline package to 1.15.1 and replace the dead GitHub zip URL with the npm tarball, which contains dist/: https://registry.npmjs.org/@gouvfr/dsfr/-/dsfr-1.15.1.tgz (dist type: tar)
• README.md (FR + EN): update both install methods and the 4 version strings, link the upstream install procedure and mention the CGU acceptance for npm/CI.
• ui_suite_dsfr.libraries.yml: license name MIT → Etalab-2.0, update url.
• ui_suite_dsfr.icons.yml: version 1.14.4 → 1.15.1 (3 icon packs).