Updated user manual to address several configuration (first pass)

Created by: jquevremont

This is a first pass to update the CVA6 user manual addressing configurations. @fatimasaleem, can you especially review the ISA extensions supported by CV32A60X in Introduction.rst? @JeanRochCoulon, can you especially review the ISA extensions supported by CV32E6?X in Introduction.rst? @AEzzejjari, I took the liberty of updating AXI_Interface.rst to align its style with other chapters (fewer indents). Tell me if it is acceptable or if I have to revert the changes.

Complete reviews are of course welcome.

What will need updates in next passes:

• CSR_Performance_Counters.rst (when Zicount in CV32A60X is confirmed)
• PMP.rst (when their presence/absence in CV32E6?X is confirmed)
• RISCV_Instructions (when the ISA extensions supported by CV32A60X and CV32E6?X are confirmed)
• Traps_Interrupts_Exceptions.rst: expected significant impact of machine-only CV32E6?X