Restructure checklist data

Unblocks some work in #940

Summary

This PR restructures checklist.json data so that each checklist category includes its own preface commentary. This allows us to loop over the checklist data to render each category in the list, resulting in a dryer template file.

Also:

• includes_checklist.njk has been renamed to _includes/checklist__task.njk.

Before

{
  "content": [
    {
      "title": "Use plain language and avoid figures of speech, idioms, and complicated metaphors.",
      "checkboxId": "use-plain-language",
      "wcag": "3.1.5 Reading Level",
      "url": "https://www.w3.org/TR/UNDERSTANDING-WCAG20/meaning-supplements.html",
      "description": "Write content at <a href='https://datayze.com/readability-analyzer.php'>an 8th grade reading level</a>."
    },
  ]
}

After

{
  "content": {
    "preface": "Content is the most important part of your site.",
    "tasks": [
      {
        "title": "Use plain language and avoid figures of speech, idioms, and complicated metaphors.",
        "checkboxId": "use-plain-language",
        "wcag": "3.1.5 Reading Level",
        "url": "https://www.w3.org/TR/UNDERSTANDING-WCAG20/meaning-supplements.html",
        "description": "Write content at <a href='https://datayze.com/readability-analyzer.php'>an 8th grade reading level</a>."
      },
    ]
  }
}