App Expert Reply:
You can “insert” a header or footer into document and can merge fields and images there. You can also insert an image merge field or any merge field into top of table though there is not a way to make the table header repeat on each page if the related list repeats multiple rows/values.
One thing to consider as possible option is allowing the page to break and controlling that and merging other tables filtering on the quote line items. Such as maybe calculating the characters or lines in the description field and using that to try to group the line items into pages and merging them in groups for each page. There may be something else in Google docs functionality to try to just get it to repeat a header for the table on each page.
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