Add detail expanders to subscription services legal terms doc

[alexgibson]

Description

The legal team have asked if we could add expanders to the VPN and Relay sections on https://www.mozilla.org/en-US/about/legal/terms/subscription-services/, in a similar way we do for the privacy notice.

We'd need to take a look at the existing markdown for these docs, and see if there's a way we can do it reliably with the current markup. If not, we could suggest some changes to the markdown to make this do-able.

[alexgibson]

Superseded by #14233