docs: add a short pg_durable / duroxide-pg disambiguator to README

[tjgreen42]

Adds a single-blockquote callout right under the opening tagline so a
reader can immediately tell pg_durable apart from
duroxide-pg, the lower-level
PostgreSQL provider for the duroxide
Rust runtime.

The earlier draft of this PR added a full "Duroxide family" overview here;
that overview is now kept only in the duroxide hub README (microsoft/duroxide#24)
so individual project READMEs stay focused on their own pitch.

Companion PRs

• docs: add a short duroxide-pg / pg_durable disambiguator to README duroxide-pg#12 — symmetric disambiguator on the duroxide-pg side
• docs: add "The Duroxide family" cross-link section to README duroxide#24 — full "Duroxide family" overview in the hub repo

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