Many couples visit lawyers for a pre- nuptial agreement before the trip to the alter, but even that foresight does not  mean a divorce will not be contentious.  A new Web site about divorce issues, www.FamilyLawSoftware.com, sees an example of the problem in a recent court case.      The case, Witowski v. Witowski, was decided on May 5 in the District Court of Appeal of Florida.  The court found that  although the prenuptial agreement listed  the husband's pension as his property, only what he owned before the marriage and the appreciation on that part were his alone.  What he and his employer contrib- uted during the marriage and the appreci- ation on those contributions were marital property, the court ruled, and the wife was  entitled to a share.      The lesson of the case, Family Law Software com- mented, is that a prenuptial agreement should be very clear on issues involving a pension, a business or any other asset that is created before the marriage and is contributed to during the marriage.      Family Law Software.com was founded by Daniel Caine and Wendell Smith, also the cre- ators of Kiplinger's TaxCut Software, now sold by H&R Block.  The site offers free help and advice in three categories of divorce problems: legal issues like custo- dy, alimony and property division; finan- cial issues like calculating child support or a pension's value; and lifestyle issues like stress and anger.  It also has links for finding professional help in each state and sells software that provides more detailed assistances.                  JAN M. ROSEN