Resolve "Comparison PDF Generation is Broken"

Closes #2665 (closed)

Enhancements in this MR:

• Comparison pages and PDF pages are both auto-generated from a single content block in features.yml! (No need to manually create a separate file to generate a PDF. No need to work with or copy/paste content into multiple blocks in features.yml)
• URLs, Comparison pages, and PDFs are now all consistent! (e.g tool first then GitLab)
• PDF formatting - features don't overlap pages breaks
• Rake script to generate PDFs finally works! (previously generated broken PDFs with missing words and text, but now includes retry logic to generate the PDF correctly.)
• The "Download" PDF button automatically appears if a PDF file exists (no need to manually set true | false)
• A fresh set of PDFs have been generated for all comparison pages!
• The handbook has been updated so everyone can contribute to comparison pages with easy step-by-step instructions! https://about.gitlab.com/handbook/marketing/website/#creating-a-comparison-page