Naming of cv.slet

Make the change: https://github.com/openhwgroup/cv32e40p/issues/833

Actions:

1. Rename cv.slet -> cv.sle
2. Add pseudo instruction cv.slet
3. Make cv.sle the preferred disassembly (ensure cv.slet is further down in the disassembly table)
4. Repeat for unsigned (cv.sletu)
5. Document pseudo instruction in official CV32E40P documentation