Have the key personnel fully participate in knowledge capture. As far as the tool to be used for capturing said knowledge, don't waste time...Just use the fast

Have the key personnel fully participate in knowledge capture. As far as the tool to be used for capturing said knowledge, don't waste time...Just use the fastest-to-set-up and the easiest-to-set-up wiki (or something similarly fast and easy). After the key knowledge has been captured, and likely after the key people are gone, you can look to see if you even need to migrate to a different platform than the original wiki. Knowledge transfer can get tricky in general...but since you're on a time constraint (key people leaving), don't waste time with UI or prettiness, etc. Just capture all the things!