TripleO says that only RHEL7 is supported

[Pete Zaitcev]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [X] This doc is inaccurate in this way: __claims that only RHEL7 is supported__
- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

-----------------------------------
Release: 3.0.0 on 2020-07-31 12:40:39
SHA: f3ae1953c282af1dc9107cf56ff2cbe9ece0edc3
Source: https://opendev.org/openstack/tripleo-docs/src/deploy-guide/source/environments/baremetal.rst
URL: https://docs.openstack.org/project-deploy-guide/tripleo-docs/latest/environments/baremetal.html

The document says:

  TripleO is supporting only the following operating systems:
    * RHEL 7 x86_64
    * CentOS 7 x86_64

Whereas for instance tripleo-heat-templates obviously has CI jobs
and bindep for CentOS 8:

https://opendev.org/openstack/tripleo-heat-templates