Broken links in customer portal docs

Some broken links leading to 404 pages are currently present in 3scale Customer portal documentation.

Get started page

The three links under "Getting started docs and guides" in the right hand side panel all get 404, here are the URLs:

• Quickstart Guide = https://access.redhat.com/documentation/en-us/red_hat_3scale/2-saas/html-single/quickstart/
• Getting Started = https://access.redhat.com/documentation/en-us/red_hat_3scale/2-saas/html-single/product/#getting-started
• Developer portal = https://access.redhat.com/documentation/en-us/red_hat_3scale/2-saas/html-single/developer_portal/

Getting Started guide

• In Chapter 1, the link to backend definition gets a 404, URL = https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.7/html/glossary/backend
• In Chapter 1, the link to product definition gets to a 404, URL = https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.7/html/glossary/product