docs: avoid framing pg-sprite as Aurora-specific

[Kiran01bm]

Summary

Docs sweep to stop framing pg-sprite as an Aurora-specific tool — it is a PostgreSQL online schema-change engine that is Aurora-aware, not Aurora-only.

Why

The doc set repeatedly branded the whole engine as "for Aurora PostgreSQL" (titles, problem statements, diagrams, the one-line pitch), while the README positions it for PostgreSQL generally — community, RDS, and Aurora. That framing undersells the tool for OSS users and contradicts the stated "Be Aurora-aware, not Aurora-only" design principle.

What

• Generalize titles, problem statements, diagram labels, and table headers from "Aurora PostgreSQL" to "PostgreSQL"; reword the pitch from "tuned for Aurora" to "Aurora-aware, not Aurora-only".
• Update the two renamed low-level-design heading anchors everywhere they are referenced.
• Add the missing "Self-managed / community PostgreSQL" row to the deployment-topology matrix (the README already claimed it; the matrix omitted it).
• Keep all genuinely Aurora/RDS-specific content unchanged: failover slot loss, rds_replication / rds.logical_replication, the version-support calendar, testing tiers, and the "Aurora PostgreSQL specifics" section.

Before                                     After
┌────────────────────────────────┐         ┌────────────────────────────────┐
│ "engine for Aurora PostgreSQL" │         │ "engine for PostgreSQL"        │
│ "a large, busy Aurora          │         │ "a large, busy PostgreSQL      │
│  PostgreSQL table"             │  ────▶  │  table"                        │
│ pitch: "tuned for Aurora"      │         │ pitch: "Aurora-aware,          │
│ topology matrix: Aurora + RDS  │         │  not Aurora-only"              │
│  rows only                     │         │ topology matrix: + self-managed│
└────────────────────────────────┘         │  community PostgreSQL row      │
                                           └────────────────────────────────┘
Aurora-specific technical content (slot loss on failover, rds_replication,
version calendar, testing tiers) is unchanged in both.

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