Add docs for windowing behavior

[zadjii-msft]

See original PR: microsoft/terminal#9118

Windowing Behavior

This setting controls how new terminal instances attach to existing windows. This propoerty is only used if the --window,-w window commandline argument is not provided. This setting accepts the following possible values:

• useNew: Create a new window, always. This is how the Terminal always behaved prior to version 1.7.
• useExisting: Create new tabs in the most recently used window on this desktop. If there's not an existing window on this virtual desktop, then create a new Terminal window.
• useAnyExisting: Create new tabs in the most recently used window, regardless of which virtual desktop the window is on.

Property name: windowingBehavior

Necessity: Optional

Accepts: "useNew", "useExisting", "useAnyExisting"

Default value: "useNew"

[!IMPORTANT]
This setting is only available in Windows Terminal Preview, version 1.7+.

[opbld32]

Docs Build status updates of commit 095bfc0:

✅ Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/customize-settings/startup.md	✅Succeeded	View

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