Yep, The Cave of Many Voices is the first episode. I think what they were trying to do was make it so that you don't necessarily have to watch them in any particular order, hence no Ruby introduction. You do get bits of her past in some episodes but I don't think there's a detailed explanation as to why she's there, outside of Chomper's parents asking her to look after him. Haven't seen the whole thing for a few years though.
And yep, the LBT channel has uploaded the TV episodes (pretty sure all of them are there but don't hold me to that).