Add primary repository license for GitHub detection

[vitormattos]

Add a top-level LICENSE containing the exact CC BY-SA 4.0 text already tracked under LICENSES/CC-BY-SA-4.0.txt.

This gives GitHub a conventional repository-level license file to detect and display while preserving the existing REUSE/SPDX multi-license model.

The README now makes the scope explicit:

• CC BY-SA 4.0 is the repository-level primary license for documentation and official artwork;
• scripts and automation remain AGPL-3.0-or-later;
• third-party fonts retain their upstream licenses;
• per-file SPDX metadata and LICENSES/ remain authoritative for exceptions.

No license assignment is changed by this PR; it only adds the conventional top-level license entry point and clarifies the existing model.

Every commit includes:

Signed-off-by: Vitor Mattos <vitor@php.rio>