fix: add preview to html's docs

[Fatme]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• Tests for the changes are included.

What is the current behavior?

Preview is not shown when tns help command is executed

What is the new behavior?

Preview is shown when tns help command is executed

Fixes/Implements/Closes #[Issue Number].