Add a couple of examples to introduction

[hpretl]

E.g., show a PCB with marked RF components from a phone teardown, or an FMCW radar board.

[EngGhaith]

What about gilbert cell down-conversion mixer shown hier:

GDS:
https://github.com/EngGhaith/mWATTBAT_RADAR_150GHz_TO_July2025/blob/main/mWATTBAT/design_data/gds/FMD_QNC_mWATTBAT.gds
Schematic:
https://github.com/EngGhaith/mWATTBAT_RADAR_150GHz_TO_July2025/blob/main/mWATTBAT/design_data/xschem/MIXER.sch
Testbench:
https://github.com/EngGhaith/mWATTBAT_RADAR_150GHz_TO_July2025/blob/main/mWATTBAT/design_data/xschem/MIXER_TB.sch