feat(docs): allow propComponents from another source

[evwilkin]

Closes #3331

This PR allows propComponents to be pulled in for components with a different source (such as a component in the next directory referencing props from a standard component) by allowing an object to be passed as a propComponent in place of the component name string.

This object should contain a component property mapped to the component name, and a source property mapped to the name of the source (ex: react). Example:

propComponents: [
  'Dropdown',
  'DropdownGroup',
  'DropdownItem',
  'DropdownList',
  {
    component: 'MenuToggle',
    source: 'react'
  }
]

[patternfly-build]

PF4 preview: https://patternfly-org-pr-3397-v4.surge.sh/v4