[Enhancement]: Add the UseASubscriptionInAJsEvent page to the FAQs menu in the docs

[bpringe]

I've seen a warning recently with a link to this page, and noticed it's not linked from the FAQs page in the docs. I only got to it by manually navigating to the URL from the warning. Should this page be linked from the FAQs page?

The URL:
https://day8.github.io/re-frame/FAQs/UseASubscriptionInAJsEvent/

The warning:

Note: The second URL in the warning is linked from the FAQs page, but the first one is not.

[dannyfreeman]

This PR would remove the warning: #754