Add design guidelines for docked navigation component

[patternfly-jira-sync]

This component needs to be documented with guidelines for usage.

The deliverable is to create a paragraph or two that describes the component and its use, and any other information necessary that its consumer should know.

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Jira Issue: PF-4195

[patternfly-jira-sync]

Closed via Jira sync - Jira issue PF-4195 is closed.