Can one create an online Minecraft world map for Minecraft Realms?
No. Realms provides easy access to the multiplayer experience, but that's all. You could see Realms as a singleplayer world, but then for multiple players, similar to the "Open to LAN" option in a singleplayer world.
If you want an online world map, you will have to create or rent a server instead. Some server hosts have this feature readily available, but I think there are also Bukkit/Spigot plug-ins that allow you to do this. Honestly I have little experience with servers, but I can tell you that realms is not what you are looking for if you want an online world map.