Improve high-level example: list rendering, SOP note, diagram path

[sorenwacker]

Three small fixes to the SEEK high-level example page:

• Add the blank line so the Measurements bullet list renders (was inline text).
• Note the experimental-design-map SOP is optional / can be added later, and link SOP to the glossary definition.
• Fix the combined-study diagram image path (../../docs/img/ -> ../../img/), which was 404ing.