[docs] Add xDiT in section optimization (#9365)
* docs: add xDiT to optimization methods
* fix: picture layout problem
* docs: add more introduction about xdit & apply suggestions
* Apply suggestions from code review
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
---------
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
