Use gerunds for task headings

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

Gerunds are "-ing" verbs. Here are examples of tasks sections from the current documentation, with suggested replacement titles:

| Current heading | Task phrasing |
| --------------- | ------------- |
| [Bare Metal Services Upgrade Guide][head1] | Upgrading Ironic |
| [Enrollment][head2] | Enrolling hardware with Ironic |
| [Node Deployment][head3] | Deploying a node on Ironic |
| [Node retirement][head4] | Retiring a node in Ironic |

Include the product name in each title so that it will be obvious in search entries. See [Search](#work-around-to-improve-search).

[head1]: https://docs.openstack.org/ironic/latest/user/index.html
[head2]: https://docs.openstack.org/ironic/latest/install/enrollment.html
[head3]: https://docs.openstack.org/ironic/latest/admin/node-deployment.html
[head4]: https://docs.openstack.org/ironic/latest/admin/retirement.html