While there isn't a single "Codex" button, you can "download" and interact with the content through these steps:
You can acquire the Arma 3 Art of War Charity Pack on Steam. It includes a special Art of War Showcase mission where you can visit an in-game gallery. download-arma-3-art-of-war-codex-online
Adds civilian clothing, gallery-themed props, and backpacks to the game. While there isn't a single "Codex" button, you
You can visit the interactive gallery in the showcase mission to view contest-winning artwork. download-arma-3-art-of-war-codex-online