Clarify use of Ibex "simple-system"

Task Description

As reported in #263, this repository has an examples/simple_system directory. Typically CORE-V core repositories do not include such examples as our members tend to have their own in-house, proprietary systems. We need to make a conscious decision about what to do about all of the "examples" in the directory of the same name.

Description of Done

One of the following:

1. Delete cve2/examples altogether and remove any references to it in all READMEs and the User Manual.
2. Update the examples to work in the CVE2 context.

Associated PRs

No response