Add a doc for setColorScheme

[zadjii-msft]

⚠️ THERE'S NO 1.3 BRANCH YET GUYS - @cinnamon-msft please feel free to re-target this PR when one gets made

This is the documentation for microsoft/terminal#6993 - the setColorScheme action.

[opbld30]

Docs Build status updates of commit e0fcc2e:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
TerminalDocs/new-terminal-arguments.md	⚠️Warning	View	Details
TerminalDocs/customize-settings/key-bindings.md	✅Succeeded	View

TerminalDocs/new-terminal-arguments.md

• Line 11, Column 1: [Warning-multiple-h1s] Multiple H1s are not allowed. You can only have one top-level heading.
• Line 11, Column 1: [Suggestion-duplicate-h1s] H1 'New Terminal Arguments in the Windows Terminal' is duplicated with other articles: 'new-terminal-arguments.md(11,1), new-terminal-arguments.md(11,1)'. First level headings must be unique within a docset. NOTE: This Suggestion will become a Warning in late September, 2020.

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