Add a new section for seismograph

[seisman]

Although the "station" section is not completed yet, I expect that the section will be very long including the following content:

• How a seismograph works
• How a seismometer works
• Instrumental responses
• network, station, channel and location
• and more

I think it's better to have separate sections for seismograph and station.

In this PR, I create a new section for seismograph and move some content from station.md to seismograph.md.