Frontmatter style guide fixes

[austin-schaefer]

• Clarifies what all the docs site frontmatter fields do
• Corrects a bunch of inaccuracy about old content types
• Adds definitions for new content types that weren't accurate
• Sweeps docs to remove a bunch of unused frontmatter fields to avoid future confusion

Reviewer: Please focus on the "Frontmatter and page templates" doc, the rest of this is just sweep work. I want to ensure the changed doc is helpful and accurate for writers

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[akristen]

This looks good to me! I reviewed the frontmatter doc and it built fine, tables rendered correctly, and the copy is good. Some nitpicky things, but overall i think it's good to merge as is :)

[nbaenam]

I fixed a typo. Other than that it's perfect!