We haven't started using it yet but we've been thinking about using the Architecture Decision Record pattern as presented in the ThoughtWorks Technology Radar [1].
The basic idea is document decisions with a specific structure and keep it close to the code. The thing is any time we can answer "why", it's a form of decision that can be documented somehow. Since it's close to the code, while coding, any search will also land on those decision if the same terms are used.
There are several tools to help with that as presented here [2] and here [3].