Tips for handing over a large amount of complex code?

Throwaway account.

I am a fairly experienced programmer who works at a large, very complex technical software project for some expensive device. Due to events which are out of the control of both the employer and me, I will leave the place in two months, and start a job in another country (yeah, Brexit sucks).

Until me giving notice, my organisation was assuming that I would be around for an unlimited time. Based on this and a considerable pressure to “get things working quickly”, there was very little focus on technical documentation. I believe the idea was mostly that “code should be self-documenting” (he), and that writing extra docs early would be a waste of time (I guess the idea was to make everything more agile, or so).

There exist some 100+ pages of API descriptions and user manuals, but very little further technical documentation which would help new people to take over the job. Now, I am facing rather desperate requests to “document everything”.

I am quite familiar with software user documentation, but I am a bit stymied about what could be the best way to hand over such a large set of code.

So, the task is to provide the most important technical documentation for the next developer to work on it in an extremely limited amount of time.

Any good suggestions?