chore(plus): add FAQ + and move {feature => features}/*

[caugner]

Summary

Fixes https://github.com/mdn/yari-private/issues/945.
Fixes https://github.com/mdn/foxfooding-mdn-plus/issues/58.

Problem

Folks will have questions about MDN Plus, but we don't have a FAQ page yet.

Solution

• Add a MDN Plus FAQ page at /en-US/plus/faq
• Link to it from the MDN Plus menu (and make that item visible to everyone)
• Rename plus/feature/* to plus/features/* to be consistent with the copy/plus structure
• Build copy/plus recursively
• Replace NaN by a proper PageNotFound error message.
• Link to it from the MDN Plus page (not for now)

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Screenshots

For non-Plus users:

For Plus users:

Feature pages (slug now have plural features):

FAQ pages:

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How did you test this change?

1. Open http://localhost:3000/en-US/plus/ and hover over the MDN Plus menu.
2. Open http://localhost:3000/en-us/plus/faq
3. Open http://localhost:3000/en-us/plus/features/notifications/

[caugner]

@Guyzeroth @schalkneethling To review, you will need to either restart yarn dev or run yarn tool spas once after checking out this branch.

[schalkneethling]

Couple of things to consider.

[Guyzeroth]

Looks really great. One small comment.

[schalkneethling]

Not something to solve now but just a general comment. The space between the page heading and the first FAQ seems very large. It feels like they are completely disconnected from each other. It also seems strange that the heading of the questions are so much larger that the page heading.

[caugner]

The space between the page heading and the first FAQ seems very large. It feels like they are completely disconnected from each other.

Good point, @schalkneethling. I reused the style from the "Learn more" pages, but feel free to improve the style there.