Updates to app dev landing page at api.openstack.org

- Offer anchor links to SDK, CLI, API
- Put rounded buttons in place
- Changes from OpenStack API to API Complete Reference
- Corrects links in the header navigation bar

Change-Id: I5267d068f005a7d7e0236baa38c3018e392d9110
Partial-Bug: 1299666
Merge "Updates to app dev landing page at api.openstack.org"