Why we do this is for "Planning before we code" and one another point is to "help the newcomers in the team". We are a team of 5-8 members. We do have API documentation and Readme included with our projects. Writing a solution document once is not a hard task. But problem facing is, It is time-consuming and hard to maintain every change we make later stages in our source-code into a separate solution document we keep. So is it worth writing a solution document?
Its depends on why you are creating the document.
To show the customer what to expect and act as a specification for the project.
To act as an instruction manual for the software that new starters can refer to.
To keep track of the requirements of the software over time, so that new requests don't remove seldom used requirements by mistake etc.
I would say that a solution doc is pretty good for 1. But people have stopped bothering with 2 all together and use unit/acceptance tests for 3.
If you find you are creating documentation because "we need documentation!" but no-one ever reads it. Then just stop writing it.