docs: add documentation authoring guideline

[gbrlcustodio]

Stacked on #597. Base is docs/architecture-principles, so this diff shows only the Phase 2 change. Review and merge #597 first.

What

Add docs/contributing/authoring.md, a short contributor guideline for how the docs/ tree is organized and where a new doc goes. It joins its siblings architecture.md and conventions.md, and is linked from docs/README.md and AGENTS.md.

Why

Phase 1 derived a set of rules for the docs tree but never wrote them down. This records them, cut to the irreducible set (mantra: less brings clarity, more brings confusion):

• Where a doc goes: audience first (contributor, consumer-by-surface, durable cross-cutting at root), then Diataxis kind, one kind per file where practical.
• Generate fast-changing reference from code (docstrings, Field(description=...), the registry, Typer help); hand-author only where no code source exists; never mix a generated table with durable prose.
• Keep it small: keep recognizable names, do not invent buckets for a few files, and send concrete migration steps to issues rather than a checklist in the doc.

The target, not the current state

The guideline describes the target the tree converges to. The current tree is mid-migration. It states the rules, not a status list, because status rots. The known divergences (generate parity.md and the config.md table; relocate ipaas.md under docs/mcp/; split the four-quadrant cli/auth.md and portal's contributor testing content; separate the contributor tails in MIGRATION.md/parity.md) are staged as separate, reviewed steps tracked in issues. Each is load-bearing in code or tests, so none happen here.

Testing

Docs-only: one new file (23 lines) plus two index rows. No code, test, or fixture path changed, so the parity test, the auth _docs.py assertions, and the CLI golden snapshot are untouched. Links resolve; no em or en dashes or curly quotes.

[gbrlcustodio]

Closing. docs/contributing/authoring.md now ships in #602, unchanged in content.

The reason is that this PR and #602 churned against each other. This PR added the file at 23 lines, and #602 added a "Decision records" section to it one commit later. A reviewer here would have read a file that the next PR in the stack immediately edited, and 25 lines across 3 files is smaller than the overhead of a separate review.

In #602 the file arrives complete, with its decision-records section already in place, alongside the records that the section describes. That pairing is the natural unit: the guideline says where a record lives, and the same PR puts the records there.

The branch stays until #597 and #602 merge.