Use a description for concept headings

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

Head conceptual sections with a descriptive title. Avoid extraneous words like "About" and "A guide to ...". "Overview" and "Introduction" are OK in headings.

Examples of conceptual sections from the current documentation, with suggested replacement titles:

| Current heading | Concept phrasing |
| --------------- | ---------------- |
| [Bare Metal service overview][con1] | Overview of Ironic |
| [Understanding Bare Metal service][con2] |Overview of Ironic (Combine with previous entry.) |
| [REST API Conceptual Guide][con3] | REST API Overview for Ironic (Only if there is more explanatory material added. Otherwise call it "REST API versioning for Ironic" since that's all it currently describes.) |

[con1]: https://docs.openstack.org/ironic/latest/install/get_started.html
[con2]: https://docs.openstack.org/ironic/latest/user/architecture.html
[con3]: https://docs.openstack.org/ironic/latest/contributor/webapi.html