docs: add the authoring guideline and the architecture decision records

[gbrlcustodio]

Why

A living document holds the rule that a contributor follows today. A record holds the reasoning that produced that rule. A living document changes as the code moves, and an adopted record does not change. The split keeps the durable reasoning out of the file that goes stale.

The docs tree also stated no rule for where a new document goes. A contributor had to infer the rule from the tree, and an inferred rule produces a misplaced document.

Outcomes

• Four decision records ship. Each one links to the living document that carries its rule, and the index maps every record to that document.
• Every record ships as proposed, because the code does not match the records yet. A proposed record stays editable in place. After adoption, a record becomes immutable, and only a later record can supersede it. A decision that reads wrong is therefore cheap to correct now.
• One record carries no living rule, because its decision stays deferred. That is a property of a deferred decision, and not a gap in the set.
• One guideline states where a new document goes, by audience and by kind. It also states the form of a convention, so the next rule arrives as a rule and not as a paragraph.
• The guideline rules on where a gap is documented. A convention states practice, so older code that predates it is legacy and the convention names no gap. The map states a claim about the code, so the map owes the reader every deviation.
• The map gains a back-link to each record that explains one of its decisions. A reader who asks why a decision holds reaches the reasoning in one step.
• The MCP reference gains the tool-design section that the map points to.
• This PR changes documentation only. No code path and no test path changed.