DOC: Update main flowchart and make modality-agnostic

[oesteban]

[jhlegarreta]

Thanks for doing this @oesteban 💯.

Some comments:

• "Estimate of the misalignment (...)" : "Estimate misalignment (...)".
• "Model" in "Drop-in Model" and "Mapping" in "Spatial Mapping" have their first letters capitalized: none of the rest of titles have them. I'd use lowercase for these as well.
• The "modality-specific structure box" is unclear to me: the distinction between "Baseline artifact" and "Artifact estimate" is unclear: the name suggest they are an artifact, but the text is about transforms. Can this be clarified/put some other way?
• The RR figure caption mentions a target but the picture contains no target text. It should be added so that it can be referred to by the caption.

[jhlegarreta]

@oesteban type-check and other errors are being addressed in PR #83.