Consulting 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 into top of table though we don’t think there is a way to make the table header repeat on each page as the related list repeats.  One thing we can try to figure 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.