[ADD] inventory: how operations affect valuation

[fw-bot]

What this PR does and why it's needed

Adds a new documentation page explaining how inventory operations affect stock valuation, giving users a single reference for how receipts, deliveries, returns, scrap, manufacturing, and manual adjustments each change a product's cost and value.

Changes

Inventory valuation

• Added a new page, "How inventory operations affect valuation", covering how stock-move operations (receipts, finished goods, customer/vendor returns, deliveries, component consumption, scrap) and non-move operations (cost updates, inventory adjustments, negative stock, valuation adjustments on a move) each increase or decrease inventory value under the AVCO/FIFO costing methods.
• Documented the tracked values on stock moves (Value, Remaining Quantity, Remaining Value) and the reported values on a product (Total Value, Unit Cost), and pointed to the Moves Analysis and Stock reports for investigating them.
• Added the new page to the inventory valuation table of contents and cross-referenced it from the valuation cheat sheet's accounting methods section and its seealso list.

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This 19.0 PR can be FWP up to master.

Forward-Port-Of: #19408

[robodoo]

[fw-bot]

@huisit this PR targets master and is the last of the forward-port chain containing:

• [ADD] inventory: how operations affect valuation #19502
• [ADD] inventory: how operations affect valuation #19501
• [ADD] inventory: how operations affect valuation #19500
• [ADD] inventory: how operations affect valuation #19499

To merge the full chain, use

@robodoo r+

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port