boards: raspberrypi: rpi_pico: Improve documentation

Reorganize for easy reference from other boards.

- Reorganized PIO documentation, creating a dedicated
  "PIO Based Features" section
- Updated "Debug Probe and Host Tools" to list supported
  debug interfaces.
- Simplified and standardized flashing and debugging instructions.
- Removed redundant J-Link debugging commands in favor of `west debug`
  usage.
- Ensured consistent use of OpenOCD and UF2 flashing descriptions.
- Added `:ref:` links for key components such as RP2040 SoC and
  Infineon CYW43439.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>