Replace Guide and Playbook content (cards) with introductions

[maggiewachs]

This is an interim step in our ongoing restructuring project to clean up some of the redundant content in these sections.

TODOs:

• [ ] Replace the Guide's landing page cards with the introduction page content (https://accessibility.civicactions.com/guide/introduction)
• [ ] Write an intro blurb for the Playbook's introduction and replace the cards with that content
• [ ] Remove the /guide/introduction page
• [ ] Redirect /guide/introduction to /guide
• [ ] Remove Guide and Playbook sections from home page