Add Experiments section — 14 RF science experiments across DVB and SDR

New section documents what you can do with the positioner beyond satellite TV:

On-board DVB (7 experiments, ready to test):
- Solar radio monitoring, rain fade radiometry, geostationary arc census,
  RFI mapping, antenna pattern measurement, DVB signal intelligence,
  differential radiometry

External SDR (7 experiments, planned):
- Hydrogen line (1420 MHz), NOAA HRPT (1.7 GHz), EME monitoring (1296 MHz),
  GPS/GNSS (1575 MHz), Iridium (1626 MHz), directional ADS-B (1090 MHz),
  Inmarsat L-band (~1.5 GHz)

Also adds shared SDR hardware setup page (feeds, LNAs, bias tee safety,
BladeRF/RTL-SDR specs), overview with status table, Experiments card on
homepage, and sidebar with nested DVB/SDR groups.

astro.config.mjs

@@ -34,6 +34,22 @@ export default defineConfig({
           label: 'Understanding',
           autogenerate: { directory: 'understanding' },
         },
+        {
+          label: 'Experiments',
+          badge: { text: 'New', variant: 'tip' },
+          items: [
+            { label: 'Overview', slug: 'experiments' },
+            { label: 'SDR Hardware Setup', slug: 'experiments/sdr-hardware' },
+            {
+              label: 'On-Board DVB (Ku-Band)',
+              autogenerate: { directory: 'experiments/dvb' },
+            },
+            {
+              label: 'External SDR (Multi-Band)',
+              autogenerate: { directory: 'experiments/sdr' },
+            },
+          ],
+        },
         {
           label: 'Project Journal',
           badge: { text: 'Living', variant: 'note' },