Create a reference section

This issue was an action item from the Ironic documentation audit.

Gather reference pages such as APIs and CLI references under a single "References" heading in the table of contents.

Reference information can be gathered in one omnibus section for the entire site, or each user guide can have its own reference chapter. You'll need to decide which makes the most sense for Ironic.

Here's a suggested outline for an omnibus reference section that includes existing reference sections in Ironic as well as reference sections that are standard across most software technical documentation:

- [Drivers, Hardware Types and Hardware Interfaces][ref1]
- [Configuration Reference][ref2]
- [API Reference (latest)][ref3]
- CLI references
  - `ironic` (link to [Ironic CLI][ironic-cli-ref])
  - `ironic-dbsync`[ironic-dbsync-ref]
  - `ironic-status`[ironic-status-ref]
- Glossary

[ref1]: https://docs.openstack.org/ironic/latest/admin/index.html#drivers-hardware-types-and-hardware-interfaces
[ref2]: https://docs.openstack.org/ironic/latest/configuration/index.html
[ref3]: https://docs.openstack.org/api-ref/baremetal/
[ironic-cli-ref]: https://docs.openstack.org/python-ironicclient/queens/cli/ironic_client.html
[ironic-dbsync-ref]: https://docs.openstack.org/ironic/latest/cli/ironic-dbsync.html
[ironic-status-ref]: https://docs.openstack.org/python-ironicclient/queens/cli/ironic_client.html