[FW][IMP] inventory: flesh out accounting

[fw-bot]

Revision of Average inventory valuation cost method.
Task ID: #3255051

Summary

Hi! I'm newly assigned to the inventory scope and wanted to update this inventory valuation doc for Odoo 14 and elaborate the accounting concepts in layman's terms. If you are able to fact-check and expand on the accounting concepts, I would greatly appreciate it!

Other changes

• Add math formulas to elaborate the journal entry calculations about inventory valuation
• Add more headings for organization (because the document got LONG very quickly)
• Renamed headings to be action-oriented and sentence case
• Add external links to related docs
• Remove instances of "I", "our", "we" and other pronouns
• Rewritten for clarity (not brevity)

Things to look out for in the review

(in order of importance)

1. What do you think about the Anglo-Saxon section (lines 186-266)
2. Does the definition of AVCO make sense?
3. Are the accounting concepts clear & accurate?
4. Do math explanations make sense?
5. Can they be shortened a little?
6. How is the organization of examples?
7. Do intros of sections seem redundant?

This PR is intended to be FW ported

Forward-Port-Of: #6458
Forward-Port-Of: #4005

[robodoo]

Pull request status dashboard.

[fw-bot]

This PR targets 17.0 and is part of the forward-port chain. Further PRs will be created up to master.

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