Skip to content should point to primary content container

[seanforyou23]

This change is necessary to create a "contract" or "handshake" if you will, between what the framework provides out of the box and what consuming applications will need to manage on their end in terms of an overall accessible experience. The overall goal is to ensure we have consistent hooks for managing focus and announcing dynamic content changes to assistive technology. By designating the main content area as the target for skip to content, we allow more advanced interactions to follow a same pattern in the code.

Related to patternfly/patternfly#2079 and patternfly/patternfly-react-seed#55

[seanforyou23]

Closed with #2519