docs: drop custom encoding section from how-to

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dfa1

docs/how-to.md

@@ -226,49 +226,3 @@ try (VortexReader vf = VortexReader.open(Path.of("future.vortex"), registry);
     }
 }
 ```
-
----
-
-## Add a custom encoding
-
-Three touch-points required:
-
-**1. Register the encoding ID** in `EncodingId.java`:
-
-```java
-MY_ENCODING("com.example.my_encoding"),
-```
-
-**2. Implement `Encoding`:**
-
-```java
-public final class MyEncoding implements Encoding {
-
-    @Override
-    public EncodingId id() { return EncodingId.MY_ENCODING; }
-
-    @Override
-    public EncodeResult encode(DType dtype, Object data) { return Encoder.encode(dtype, data); }
-
-    @Override
-    public Array decode(DecodeContext ctx) { return Decoder.decode(ctx); }
-
-    private static final class Encoder { /* ... */ }
-    private static final class Decoder { /* ... */ }
-}
-```
-
-**3. Register via `ServiceLoader`** — add the fully-qualified class name to:
-
-```
-META-INF/services/io.github.dfa1.vortex.encoding.Encoding
-```
-
-The encoding is then picked up automatically by `EncodingRegistry.loadAll()`.
-To register it only for a specific reader without touching the global registry:
-
-```java
-EncodingRegistry registry = EncodingRegistry.loadAll();
-registry.register(new MyEncoding());
-VortexReader vf = VortexReader.open(path, registry);
-```