I don't know of any method to automate the whole process and record the net number of words added to a document in a particular day.
That said, you could create a second document in Numbers, place a single table in that document, and use it to record the date and the current word count in the Pages document at the end of each day.
Include a Start row with no date, and 0 entered in the word count column.
The selected cell (C3) contains the 'single line' formula shown below, entered in the selected cell (C3), and filled down for as many rows as you need. The core formula here is B3-B2, which subtracts the precious day's 'total word count' in B2 from today's total word count, entered in B3.
The second ('two line') formula is placed in cell D3, and filled fown to the same rows as the C3 formula. The core formula here calculates the average words per day by by dividing the current total word count (B3) by the number of days worked ( row()-2 ).
The IF part of the formula, which is the same for both these formulas gets the value from the cell in tis row of column B. If the cell is empty ( "" ) the formula returns a null sting ( "" ) which displays as an empty cell. f the cell is not empty, it will contain a word count, and IF calls the core formula to calculate the 'words today' value or the average wpd number for that day.
The current word count can be shown at the bottom of the page by going to the View menu and choosing Show word count, or by pressing shift-command-W to toggle between showing and hiding that count.
Regards,
Barry