Removing extension interface section from Documentation and link to the official one

Created by: davideschiavone

The cv32e40x documentation regarding the core-v-x interface should only contain references to the official spec: https://docs.openhwgroup.org/projects/openhw-group-core-v-xif/ .

In addition, it may specify what is supported and what not, which signals are hardwired, etc which are related to implementation details