Add preview text and re-add missing enableReadOnlyMode docs

[nguyen-dows]

This PR adds the below preview text to instances of enableReadOnlyMode and disableReadOnlyMode in our docs (specifically in actions.md and panes.md)

`> [!IMPORTANT]
> This feature is only available in [Windows Terminal Preview](https://aka.ms/terminal-preview).

This PR also adds the below preview text to the activeOnly value for the 'showCloseButton' property in TerminalDocs/customize-settings/themes.md.

> [!IMPORTANT]
> The `"activeOnly"` value is only available in [Windows Terminal Preview](https://aka.ms/terminal-preview).

Lastly, this PR re-adds the missing enableReadOnlyMode docs in TerminalDocs/customize-settings/actions.md that was accidentally removed in PR #660

[learn-build-service-prod[bot]]

Learn Build status updates of commit c34a710:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/customize-settings/actions.md	:white_check_mark:Succeeded	View
TerminalDocs/customize-settings/themes.md	:white_check_mark:Succeeded	View
TerminalDocs/panes.md	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

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