Update Demos & Tools for Sheets Add On.

• Move the spreadsheet add on page to the GA4 pages directory. The page will be created by Gatsby at the path /ga4/spreadsheet-add-on/
• Ensure spreadsheet add on displays in the menu side bar.
• Fix the links in the spreadsheet add on page.
• If user visits legacy UA page directly, redirect to corresponding GA4 page
• Remove setGAVersion function that included UA/GA4 redirect logic but wasn't being used.
• Remove gaVersion useMemo hook which was calculating the value of gaVersion based on whether the path included /ga4/.
• Update the Layout component to always use the GA4 version.