Put "Using Rails for API-only Applications" in table of contents

[piotrj]

When I was looking for more info regarding this the only way I ended up
on that page was by googling something along the lines of "rails new
api" (as I wanted to find out what are the proper parameters when
generating api app). I think it's beneficial to have that page in table
of contents.

[rails-bot]

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[maclover7]

Weird this wasn't already included. In the future, please include [ci skip] in your commit message to avoid running the entire test suite for documentation changes.

[maclover7]

👍 Thank you for your contribution!

[maclover7]

❤️ 💚 💙 💛 💜