Rewrite the landing page

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

Put Getting Started information on the landing page. ([This work is already underway!](https://review.opendev.org/c/openstack/ironic/+/922484?tab=comments))

Here are some suggestions for the "getting started" part of the landing page:

Write a short introduction and a map to which Getting Started guide is appropriate – something like the table below.

| If your goal with ironic is: | Start here: |
|-------|---------------|
| Understand and evaluate ironic for my organization | Technical overview |
| Install ironic standalone for a small number of nodes | Standalone Getting Started guide |
| Use Ansible to install and configure nodes | Bifrost Getting Started guide; Bifrost documentation |
| Use ironic with other OpenStack components | Integrating ironic with OpenStack Getting Started guide; Nova documentation; Neutron documentation; etc. |