Add PL/php logo assets and use them in the docs

CHANGELOG.md

@@ -20,6 +20,8 @@ and the project aims to follow [Semantic Versioning](https://semver.org/).
 
 ### Added
 
+- **Project logo and brand assets.** Light/dark PL/php logos and a square icon
+  under `doc/assets/`, wired into the README and language reference.
 - **Error CONTEXT lines.** Messages raised while PL/php code runs carry a
   `CONTEXT: PL/php function "name"` line (or the anonymous-block/compilation
   variants), like every other procedural language.

README.md

@@ -1,6 +1,9 @@
 <div align="center">
 
-# PL/php
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="doc/assets/plphp-logo-dark.svg">
+  <img src="doc/assets/plphp-logo.svg" alt="PL/php — Procedural Language for PostgreSQL" width="460">
+</picture>
 
 **Write PostgreSQL functions, triggers, and procedures in PHP.**
 

doc/plphp.md

@@ -1,3 +1,10 @@
+<div align="center">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="assets/plphp-logo-dark.svg">
+  <img src="assets/plphp-logo.svg" alt="PL/php" width="360">
+</picture>
+</div>
+
 # PL/php Language Reference
 
 PL/php lets you write PostgreSQL functions and triggers in PHP. This document