Help users understand how to add WebTunnel bridges

@ebanam has shared that both desktop and Android users often contact user support to ask how to add WebTunnel bridges, despite the fact that they are added the same way as any other bridge. @sajolida speculates that this could be due to Tor Browser's UI not mentioning WebTunnel anywhere.

We could make it more obvious that users can paste all bridge types into the Add new bridges… dialog by explicitly naming the PTs that are supported, i.e. obfs4, Snowflake, meek-azure and WebTunnel. However we should keep in mind that vanilla bridges (and in future, Lox invites) can be added via this dialog too.