[doc] Document support Ibex configurations

[GregAC]

Document what it means to be an 'supported' configuration along with the current supported configurations and how the ibex_config.py utility works.

Part of the configuration documentation can be autogenerated from the ibex_configs.yaml file. This will require adding a description field to each configuration.

[vogelpi]

While going through old issues and checking the documentation, I noted that the the first part of the issue (document what it means to be a "supported" config and currently supported configs) is already documented in the README.md but I couldn't find information about this in the actual documentation.

So, it looks like two things remain:

• [ ] Document currently supported configs and what it means to be supported to the actual documentation.
• [ ] Document how the ibex_config.py utility works.