Rework ARK RTK GPS and ARK X20 RTK GPS docs to match other GPS product pages

[dakejahl]

The IMU CAN param docs added in #24 cover every CANnode-based GPS/Flow product except ARK RTK GPS and ARK X20 RTK GPS. Both pages need a structural rework before the IMU param rows fit naturally, so they were intentionally deferred.

ARK RTK GPS — `gps/ark-rtk-gps/px4-instructions.md`

Currently a one-line stub that just links to the upstream PX4 documentation. Should be expanded to match the structure used by ARK GPS, ARK G5 RTK GPS, ARK MOSAIC-X5 RTK GPS, and ARK TESEO GPS:

• Hardware setup / wiring blurb
• `Required Parameters` table (`UAVCAN_ENABLE`, `EKF2_GPS_CTRL`, etc.)
• `Optional Parameters` table (`EKF2_GPS_POS_*`, `CANNODE_TERM`, `CANNODE_PUB_IMU`, `UAVCAN_SUB_IMU`)

ARK X20 RTK GPS — `gps/ark-x20-rtk-gps.md`

Currently a single landing-page-style file that mixes firmware downloads, pinouts, and 3D models. Needs:

• Restructure into a folder with a landing page + `px4-instructions.md` + `ardupilot-instructions.md` subpages (same pattern as the recent ARK TESEO GPS rework in 4a425f1)
• A new ArduPilot instructions page (none exists today)
• Once split, the new PX4 page should include the standard `Required` / `Optional` parameter tables — including `CANNODE_PUB_IMU` and `UAVCAN_SUB_IMU` for the onboard ICM-42688-P
• Update `SUMMARY.md` to reflect the new folder layout

Reference rows to add once reworked

```
| CANNODE_PUB_IMU | Set to `1` on the GPS via the DroneCAN GUI Tool to publish the `RawIMU` messages on the CAN bus |
| UAVCAN_SUB_IMU | Set to `1` on the autopilot to subscribe to DroneCAN `RawIMU` messages. Requires `CANNODE_PUB_IMU` to also be set on the GPS |
```