Update introduction to diffusion MRI lesson

[Bradley-Karat]

Added in and expanded upon the current information in the introduction to diffusion MRI lesson. This includes changes to improve clarity and the logical flow of the lesson. Also changed the first figure to something that is a bit more clear (i.e. easier to understand the signal change via gradient direction change).

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[Bradley-Karat]

Changes to preprocessing module accidentally got added here, closing this pull request and will re-submit only changes to introduction