Refresh concept pages and add landslide monitoring with GNSS

[andygrillo]

What changed

• New: Landslide Monitoring with GNSS. Why RTK is needed, the base as the reference on stable ground, survey and baselines, reading scatter (up to about 2 cm), noisier vertical, what RTK Fixed does and does not prove, site factors (sky view, reflections, mounting), the 6-hour reading interval, and interpreting results with a geotechnical specialist. Slide has no alerts, stated plainly.
• Data Quality: new "Data quality with Grillo sensors" section: health and connection history for everyone, waveforms with SISTEM, NTP timing and the mis-timed first minutes, raw counts.
• Connected Sensor Networks: Grillo One alongside Pulse, a local radio mesh option (Slide), and a rewritten Grillo architecture section covering data routing and Slide.
• Seismic Networks / Sensor Placement: Grillo One as a densification station; waveform checks need SISTEM or your own system; links to One and Slide installation.
• Earthquake Early Warning unchanged.

npm run build passes.

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