The CVE2 cores do not have their own section header

Once #599 is merged in, docs/cores.rst, the CVE4 Series of CORE-V Cores and CVA6 Series of CORE-V Cores will have their own section header, but not the CVE2 Series. It makes it look as if the RTL In Development CVE2 User Manuals are part of the CVA6 Series.