Add 'WebTunnel' to the Glossary

Let's add WebTunnel to the glossary.

ref. https://blog.torproject.org/introducing-webtunnel-evading-censorship-by-hiding-in-plain-sight/

WebTunnel is a censorship-resistant pluggable transport designed to mimic encrypted web traffic (HTTPS) inspired by HTTPT. It works by wrapping the payload connection into a WebSocket-like HTTPS connection, appearing to network observers as an ordinary HTTPS (WebSocket) connection. So, for an onlooker without the knowledge of the hidden path, it just looks like a regular HTTP connection to a webpage server giving the impression that the user is simply browsing the web.