Provide better theoretical background across the lesson

[jhlegarreta]

After today's dry-run, I feel that the lesson would greatly benefit from a few improvements that could allow instructors to provide a clearer message concerning some key aspects with the help of some additional figures and some expanded explanations. Especially, if time does not allow to follow the episodes by typing all commands, this would also enable summarizing better the contents of an episode, and hopefully allow learners to grasp better the underlying ideas.

Specifically, it might be beneficial to:

• Add an isotropic vs. anisotropic/random walk picture in the introduction episode (without overlapping with/harm to the eigenvalues/eigenvectors picture in the DTI episode). Probably accompanied by/requiring a better explanation.
• Improve the explanations/add a short theory section about the dMRI physics and what it adds to a regular MRI (main magnetic field, RF pulses, gradients, briefly mentioning the sequences, hopefully with some pictures, briefly mentioning b-tensor encoding, QTI, etc. or adding a picture that shows the tensors and sequences/trajectories). It can be good to look at the sMRI lesson acquisition and modalities episode to avoid repeating things and to be able to assume that some aspects are already known/build bridges across the two lessons.
• Add the definition of the b-value, including its formula. Would maybe require briefly explaining where it comes from/showing the formula's around. Can also lead to linking these to the local models, EAP, FOD, etc.
• Improve the explanation of what the B₀ volume is.
• Improve the explanation about different sources of noise/distortion, hopefully adding pictures to show what such distortions look like/how to distinguish them (magnetic susceptibility, Eddy currents, magnetic field inhomogeneities, ghost artifacts, motion).
• Add a picture of how the FA changes with (an)isotropy.
• Mention the asymmetric ODFs, and why only symmetric ODFs are used customarily.
• Add pictures of the local model glyphs.
• Add an actual picture that shows better where a DTI model would fail (e.g. a crossing fiber).
• Add a picture that clearly shoes the difference of a det vs. prob tracking.
• Add further extra contents about downstream tasks/derivatives (tractography enhancement, bundling, tractometry, etc.).
• Add some extra contents about microstructure models.

Best would be to fix them in separate PRs so that contributions can be done more easily.

Related to #28 and #37.