Reorganize repository

Created by: cathales

Because of CVA6-related code in core-v-verif, modifications in CVA6 often require a modification in core-v-verif. These pairs of PRs are time consuming for maintainers and require a complex continuous integration. This repository interaction is complex for newcomers.

This PR:

1. moves the cva6/ folder from the core-v-verif repository as a verif/ folder in the CVA6 repository, keeping the history alive (with the same commit hashes, to make it easier to browse history across repositories).
2. adds core-v-verif as a git submodule in verif/core-v-verif/ (there are already many git submodules so it does not imply more knowledge for newcomers).

Todo before merging:

• Update documentation (eg: simulations are run from the CVA6 repository)
• Rebase for both repositories just before merging the PR.

Expected work after merging:

• Additional cleanup / refactoring to eventually remove the complex mechanisms that were used before (mainly in CI scripts)