boards: riscv: longan_nano: Update document

Improve documentation

- Add a photo of the board
- Add hardware information from the board manual
- Fix options for build (set OPENOCD_DEFAULT_PATH to make it
  valid for also windows environment)
- Improve the build instructions to make it easier to understand

Signed-off-by: Yuichiro SAGISAKA <yu.sagisaka@fujitsu.com>