Epic: Reorganize 'plugin-tools' site to match current site structure plan

[josmperez]

This epic covers for the entire site of plugin-tools:

• Reorganize 'plugin-tools' site to match the current site structure plan
• Create redirects from old location of files so that nothing breaks
• Update links within plugin-tools itself so that they go to the new file locations
• Fix any typos or minor style issues that may be noticed during this process.

  
  ### Tasks

• [x] Chunk 1: Reference directory
• [x] Chunk 2: Introduction/Key Concepts directories
• [x] Chunk 3: Create a Plugin directories
• [x] Chunk 4: Other directories

[josmperez]

Reorg complete.