Reorganize plugin-tools site based on new design plan

[josmperez]

Fix problems with less than ideal plugin-tools organization. Incorporate feedback received in Q1 from many stakeholders, external and internal to the plugins team and approved in a design plan.

This process includes the following:

• Move or rename folders and files
• Fix broken links on site caused by move/renames
• Fix broken links on other sites, principally grafana/grafana and plugin-examples
• Create new topic placeholders for new topics, make edits and/or assign to developers to make edits
• Test all changes through build process on Docker site and dev site

  
  ### Tasks

• [x] Reorganize 'reference' section
• [x] Reorganize 'introduction' section
• [ ] Reorganize 'create-a-plugin/develop-a-plugin' section
• [ ] Reorganize 'create-a-plugin/extend-a-plugin' section