[SPARK-55277][SQL] Add protobuf_funcs group for Protobuf SQL functions

### What changes were proposed in this pull request?

Similar to how Avro functions have their own `avro_funcs` group, Protobuf functions (`from_protobuf`, `to_protobuf`) should have their own `protobuf_funcs` group instead of being grouped under `misc_funcs`.

This improves consistency with how other format-specific functions are organized and makes the documentation clearer for users.

Changes:
- Move `from_protobuf` and `to_protobuf` from `misc_funcs` to `protobuf_funcs`
- Add `protobuf_funcs` to the groups set in `gen-sql-functions-docs.py`

### Why are the changes needed?

Consistency with `avro_funcs`, `json_funcs`, `csv_funcs`, `xml_funcs` groupings.

### Does this PR introduce _any_ user-facing change?

No functional changes. The only difference is in how functions are grouped in documentation.

### How was this patch tested?

Existing tests.

### Was this patch authored or co-authored using generative AI tooling?

Yes, GitHub Copilot was used to assist with this change.

Closes #54059 from yaooqinn/SPARK-55277-protobuf-funcs-group.

Authored-by: Kent Yao <kentyao@microsoft.com>
Signed-off-by: Kent Yao <kentyao@microsoft.com>

Author: yaooqinn

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/toFromProtobufSqlFunctions.scala

@@ -58,7 +58,7 @@ import org.apache.spark.util.Utils
     To deserialize the data with a compatible and evolved schema, the expected Protobuf schema can be
     set via the corresponding option.
   """,
-  group = "misc_funcs",
+  group = "protobuf_funcs",
   since = "4.0.0"
 )
 // scalastyle:on line.size.limit
@@ -195,7 +195,7 @@ case class FromProtobuf(
       > SELECT _FUNC_(s, 'Person', '/path/to/descriptor.desc', map('emitDefaultValues', 'true')) IS NULL FROM (SELECT NULL AS s);
        [true]
   """,
-  group = "misc_funcs",
+  group = "protobuf_funcs",
   since = "4.0.0"
 )
 // scalastyle:on line.size.limit

sql/gen-sql-functions-docs.py

@@ -36,7 +36,7 @@
     "bitwise_funcs", "conversion_funcs", "csv_funcs",
     "xml_funcs", "lambda_funcs", "collection_funcs",
     "url_funcs", "hash_funcs", "struct_funcs",
-    "table_funcs", "variant_funcs"
+    "table_funcs", "variant_funcs", "protobuf_funcs"
 }