Add information on junctions to wire documentation

A while ago, I added junctions to some wires. Today, I needed to do it again, and couldn't remember what keys to hold down while dragging. The documentation doesn't mention junctions at all.

Once I found out that they were called junctions (searching for "node red split wire" doesn't produce useful results) I found something in the 3.0 release notes but it took over 10 minutes in all.

Could something be added to the documentation covering this feature? I can submit a PR for the text, but I'm not really in a position to generate helpful images at the moment unless I just lift the ones from the release notes.

從 wire 文件頁面開始，並將其與連結的 Node-RED 3.0 junctions 發行說明進行比較。記錄如何建立 junctions，包括拖曳時需要按住的按鍵；如果有可用的圖片，也請加入有幫助的圖片。文件應讓使用者無需在其他地方搜尋即可發現並使用此功能。