Start with the data flow: what interacts with user, what interacts with other systems, what exposes a remote API, what components pass data where, and what data

Start with the data flow: what interacts with user, what interacts with other systems, what exposes a remote API, what components pass data where, and what data structures are used between them. You'll also need to document what processes and modules/subsystems are there (cron job? job queue? other daemon/worker of sorts? and then layout in the code). And build/deployment process, especially with today's CSS and JavaScript compilers.