Improve the MR modalities figure

[jhlegarreta]

The MR modalities/MR scan types figure:
https://github.com/carpentries-incubator/SDC-BIDS-IntroMRI/blob/gh-pages/fig/mr_scan_types.png

is great to show how some modalities can be used. However, I'd dare to say that "DTI" is not a modality in its own, but rather a reconstruction method or model used on dMRI or DWI images. Also, although showing the tractography result is very appealing visually, I'd show a raw dMRI image. Maybe some derivatives can be added for each modality later (like some segmentation on the T1w, the FA and tractography on the dMRI images, etc.).

Not sure where this figure is being displayed now, but it was being displayed in the Neuroimaging Fundamentals episode in PR #23 (https://deploy-preview-23--carpentries-intromri.netlify.app/introduction/index.html).

Also, it looks like there is some overlap with the sMRI episode concerning that passage:
https://carpentries-incubator.github.io/SDC-BIDS-sMRI/01-Image_Modalities/index.html

Best would be to use either the same figure, or maybe slightly different ones but two in which the modalities are consistent, and possibly mentioning which other modalities exist as of today/cross-referencing the episodes across lessons.

Thanks for the effort with this lesson 💯.