Remove some high-level contents from seismic-wave section

[seisman]

Although dispersion is a important property of surface waves, it's likely that we won't talk about dispersion in our seismology exercise. Thus, I prefer NOT to talk too much about surface wave dispersion here.

[github-actions[bot]]

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• Preview: https://seismo-learn.org/sitepreview/seismo-learn/seismology101/seismic-wave/index.html
• Commit hash: c610525c4961b46a7b82c3bf89bba2544b7aab9e

[core-man]

Although dispersion is a important property of surface waves, it's likely that we won't talk about dispersion in our seismology exercise. Thus, I prefer NOT to talk too much about surface wave dispersion here.

How about moving it to seismology (https://seismo-learn.org/seismology/) first before removing the content in the exercise chapter.