docs: document A4 and 16:9 manual formats

[vitormattos]

Summary

Clarifies in the repository README that the LibreSign brand manual is published in two layouts from the same canonical source.

Changes

• replaces the single A4-only manual link with an explicit Brand manual formats section;
• lists both A4 portrait and 16:9 widescreen editions;
• links directly to both assets from the mutable latest release;
• explains the intended use of each format;
• makes it explicit that both PDFs come from manual/main.typ and share the same brand rules;
• clarifies that the 16:9 edition is a widescreen reading/presentation layout, not a separate manual or abridged speaker deck.

This aligns the root README with the build/release behavior already implemented by CI and documented under manual/README.md.