Document unbound command

[andreykaipov]

The previous version of the docs that were removed in PR microsoft/terminal#7649 had detailed documentation and motivation for the "unbound" action:

Unbinding keys

If you ever come across a key binding that you're unhappy with, it's possible to
easily change the keybindings. For example, vim uses Ctrl+^ as a
binding for "switch to previous buffer", which conflicts with the Terminal's
default keybinding for "open a new tab with the sixth profile". If you'd like to
unbind that keybinding, and allow the keystroke to fall through to vim, you can
add the following to your keybindings:

{
   "command" : null, "keys" : ["ctrl+shift+6"]
},

And mentioned further in another section:

If you'd like to unbind a keystroke that's bound to an action in the default
keybindings, you can set the "command" to "unbound" or null. This will
allow the keystroke to fallthrough to the commandline application instead of
performing the default action.

The poor unbound action has now been relegated to a tiny section on this page without even an example for it:

https://docs.microsoft.com/en-us/windows/terminal/customize-settings/actions#unbind-keys

😢

[mattwojo]

Finally got around to adding an update with this additional info / context and giving the poor relegated unbound action some love! <3 Thanks for calling this out @andreykaipov! :)
69655c9