message: Add XLA logo to its documentation page
diff: # XLA Overview
  +<div style="width:50%; margin:auto; margin-bottom:10px; margin-top:20px;">
  +<img style="width:50%" src="/images/xlalogo.png">
  +</div>
  +
  > Note: XLA is experimental and considered alpha. Most use cases will not
  > see improvements in performance (speed or decreased memory usage). We have
  > released XLA early so the Open Source Community can contribute to its
  