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Spoilers as well! ; ) 

Hi! Yes, you can go below! There's a separate quest related to the dolmen, but it's not currently a part of the demo. During this quest you'll get to know that you should look for a "trapdoor". If you'd pick a scholar, your could also read the inscription to find a direct clue. 

The arrow that you can currently see is, well, vague, since you can't read. If you'd like to go deeper, you should look for a "hatch" or "trapdoor". Nevertheless, you're almost at the very end of the current stage of the demo version.