Document "Locked Panes" before they go live in 1.7

[Don-Vito]

We need to write a documentation for microsoft/terminal#6981 (implemented by microsoft/terminal#8867).

As @cinnamon-msft, mentioned here:

I could see this in the Panes page of our docs, and also in our Tips and Tricks page. :)

[mattwojo]

Looks like this is resolved. Please reopen if this isn't the case.