Add extensions link back to User Guide

[hxlnt]

After webhintio/hint#2837 was merged, the link to browser extensions disappeared from the User Guide sidebar. Guessing something with how the the static-site generator creates the sidebar needs to be adjusted.

[antross]

Actually #793 is probably the culprit here. We have to publish the extension package to NPM now for it to get picked back up.

[sarvaje]

Yep, we need to publish the extension to NPM to get it back.

[hxlnt]

Cool--closing this out then.