Add more general how-to-use guidance to liesel docs

[jobrachem]

Maybe also one general suggestion would be to create more general tutorials on liesel, i.e. connecting nodes, building the graph and how liesel works under the hood. While model specific tutorials are always helpful and get you going fast, they don't give you a deep understanding on how a software package works. For me, this always becomes prominent as soon as I need to deviate from the model setup in the tutorials, even though your documentation certainly provides assistance. Of course in some of the tutorials you also touch up on these points.

Originally posted by @Seb-Lorek in https://github.com/liesel-devs/liesel/discussions/105#discussioncomment-7421477

I think this could indeed be quite beneficial. More specifically, I think we could use something like:

• [ ] #124
• [x] #125
• [x] #126
• [x] #127
• [x] #128