[ADD] manufacturing: work in progress

[huisit]

documentation task card: https://www.odoo.com/odoo/my-tasks/5868698

• add work-in-progress costs article since WIP is not covered yet in our documentation; supplements manufacturing costs article
• focus on workflow of creating WIP entries in MRP to stay in app scope; add link to accounting article if/when that is published

This 19.0 PR can be FWP up to master.

[robodoo]

[huisit]

@jero-odoo this is ready for peer review!

there are a few things i'd especially love to get feedback on:

• the first section title
• whether that first section should have subsections for consumed + real cost
• whether the manage wip entries parent heading is necessary (vs. workflow sections top-level)
• numbered lists! thought this might be suitable since it's accounting-adjacent, they use OL regularly