Add input devices

content/10_input_devices.md

static/designs/10_audio_cutout.png

static/designs/10_audio_input.brd

static/designs/10_audio_input.sch

static/designs/10_audio_top.png

static/designs/10_sampler.c

static/img/10_amplification.jpg

static/img/10_board.png

static/img/10_isp_reuse.jpg

static/img/10_nonlinear.png

static/img/10_oscillations.png

static/img/10_phono_input.jpg

static/img/10_schematic.png

static/img/10_square_waves.png