docs(outbox): document single-commit vs 2PC consumer pattern

[asawicki]

What

Adds a Consumer Business Writes: Single-Commit vs 2PC section to the published Outbox article (docs/articles/outbox.md).

The recent outbox changes (sharing the middleware's pinned connection via IOutboxConnectionAccessor so a consumer's business write commits single-phase with the outbox/inbox writes) were documented in the EF package README, but the published DocFX article was never updated. This closes that gap.

Why

• The DocFX Outbox article never documented a 2PC requirement to begin with, so nothing there was incorrect — but it also never described the new, recommended single-commit pattern.
• A consumer that persists business state through its own DbContext needs to know how to avoid escalating to a two-phase (prepared) commit, which on PostgreSQL fails by default (max_prepared_transactions = 0).

Details

The new section explains:

• Two physical connections → two-phase (prepared) commit, with the 55000: prepared transactions are disabled failure mode and its cost.
• One shared connection → single-phase commit (recommended) via IOutboxConnectionAccessor, removing the max_prepared_transactions requirement.
• A wiring example using the (IServiceProvider, DbContextOptionsBuilder) overload, aligned with the article's existing TransferDbContext example.
• A pointer to the samples that use this pattern.

Docs-only change — no code or behavior affected.