This seems to be the most official information that is arround dp-alt-mode-adapters.org · master · Librem5 / firmware-tps6598x-nonfree · GitLab (found here: Docked mode · Wiki · Librem5 / Librem 5 Community Wiki · GitLab)
Does sell already: https://shop.puri.sm/product-category/phones/accessories-phones/
…and not supported at all - if I remember correctly. I’ll add some information or reference about it later on the wiki.
Edit: I remember @guido.gunther mentioning this somewhere, but this might as well be a memory corruption on my side, because FAQ says hdmi is supported. I’ll need to ask @guido.gunther. Put the references on the wiki page.
The Librem5 supports usb -c display port alternate mode. If (and usually) there is a hdmi connector on a dock that works with the L5 the dock takes care of converting the signal from display port to hdmi.
HDMI alternate mode is as far as I know not supported by the Librem5.