Refactor plugin section of the pulp3 documentation
Based on the proposed doc structure  the Plugins section of the documentation should be reorganized like the following:
- Brian's plugin table
- Link to plugin writer's guide
Plugin_API Reference (autodoc)
Plugin Writer's Guide
- Writing your first plugin
- Generating a CLI
- Releasing your plugin
The Landing Page should contain a summary of what belongs in this section. Each separate page should be stubbed out.
The current Plugin_API section should be renamed to API_Reference and moved to the top level section of the pulp docs.