Define API for documentation framework

[dlabaj]

Description

Add API for how react, core, or an extension would integrate the content from the corresponding repo into the documentation site.

To Do

• Any external APIs that are needed to get react, core, or extension content into the Patternfly Site has been defined.
• Code has been added to support APIs
• Build has been updated to pull in external content using the APIs
• Verify the API works with core in a separate branch. This does not need to be checked in, just verified.
• Code has been tested and unit tests have been written.
• CI / CD integration