[IMP] inventory: replace purchase/manufacturing lead times with horizon days

[huisit]

documentation task card: https://www.odoo.com/odoo/project.task/6046072

key change:

• remove references to purchase security, manufacturing security.

misc. changes:

• update lead time diagrams to reflect removal of purchase/manufacturing security lead times.
• update text examples to reflect removal of purchase/manufacturing security lead times.
• replace outdated GUI screenshots.
• rename & add custom anchors for consistency.
• add missing anchors to types of lead times section.
• reorder sections and types of lead times list to reflect order of operations in JIT logic.
• clarify days to prepare manufacturing order logic, since it doesn't appear in the diagram.

note:

• although this PR touches a lot of the article, it is not a full article refresh; planning to scope out remaining work next week.

This 19.0 PR can be FWP up to master.

[robodoo]

[huisit]

@theRealThagomizer this is ready for review!

[Felicious]

due to the complexity of the topic, im upping the scope from 3 to 5 even though it's not a full article refresh (: