WASHINGTON — Sandra Day O’Connor, the first woman on the Supreme Court, announced Tuesday that she has the beginning stages of dementia, “probably Alzheimer’s disease.” The 88-year-old said in a public letter that her diagnosis was made some time ago and that as her condition has progressed she is “no longer able to participate in public life.” “While the final chapter of my life with dementia may be trying, nothing has diminished my gratitude and deep appreciation for the countless blessings in my life,” she wrote.