Add tutorial-style notebooks to explain ADC theory

[mfherbst]

Sections for 0_basics:

• [x] Overview
• [x] Backends
• [ ] Theory (just a bit about the matrix structure, the ISR, similarity to MP, CC and CI ... no details just a working man's knowledge)
• [ ] Tweaks (convergence tolerance, subspace, threading, ...)
• [ ] Properties (What sort of properties adcc can compute, just an overview no details)
• [ ] Core excitations (A chapter focusing on CVS and core-excited states, XES, XAS, core space setup etc)
• [ ] Embedding and solvation

Other sections / topics:

• [ ] Dataframe export and visualisation
• [ ] In-depth discussion of ADC theory including equations (using the einsum and Tensor interface of adcc to teach ADC methods to newcomers.)
• [ ] Custom tensors and implementing equations (explain einsum etc.)

[mfherbst]

While we're on it, we should restructure the example files. Best would be to restructure according to topics (properties, plotting, X-ray) and put a readme in each subdirectory, which can be easily seen on github. From the adcc docs one could just link directly to these folders.