Raise screenshot error earlier when page is closed

[wintonzheng]

[!IMPORTANT] Raise FailedToTakeScreenshot in take_screenshot() if page.is_closed() in page.py and add TOC to workato.md.

• Behavior:
  • In take_screenshot() in page.py, raise FailedToTakeScreenshot if page.is_closed() before attempting to take a screenshot.
• Documentation:
  • Add table of contents to workato.md.

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