LiteX on Linux example lacks documentation

[marcmerlin]

https://symbiflow-examples.readthedocs.io/en/latest/building-examples.html
is confusing.

1. it has this bit about
   cd litex-boards/litex_boards/targets; ./digilent_arty.py --toolchain symbiflow --cpu-type vexriscv --build
   which does not say where this is supposed to be installed/built

2. lower down it says TARGET="arty_35" make -C linux_litex_demo
   this command works fine, and so does the openocd push (without the above because it didn't work for me as per /opt/symbiflow/xc7/install/bin/symbiflow_synth: line 93: prjxray-config: command not found + "ERROR: No such command: tcl" #154 )
   instructions do not say that this time serial terminal is at 115200 (different speed from the previous example). I guessed the speed eventually and got

Unable to download Image over TFTP
Network boot failed
Unable to download Linux images, falling back to boot.bin
Unable to download boot.bin over TFTP
Network boot failed
No boot medium found

--============= Console ================--
litex>

this is understandable as it's missing the linux image from the previous step that failed I assume, although the previous step seems very complicated compared to grabbing a pre built linux kernel and initrd, so I'm not sure how this was supposed to work