add an extendedData field to tables

`extendedData` provides supplementary per-cell data for tables. It's
indexed with a generated string key with the format 'X_Y', as described
in its "description" field. For the most part this can be omitted, but
here it introduces the 'colspan' and 'rowspan' properties, with the
following semantics:

- the default value of each of these is 1. the property will be absent
  in this case.
- if all values in an `extendedData` object are the default, it will not
  be included in the Render JSON.
- a value greater than one has the same semantic meaning as the
  attributes of the same name on the HTML `tr`/`th` elements - that
  this cell should fill in the space of more than one column and/or row.
- a value of zero means that the cell should not be rendered in HTML
  because another cell has a span that overwrites it.

Author: QuietMisdreavus

Sources/SwiftDocC/SwiftDocC.docc/Resources/RenderNode.spec.json

@@ -839,11 +839,29 @@
                             }
                         }
                     },
+                    "extendedData": {
+                        "type": "object",
+                        "description": "Additional data that can be applied per-cell. Property keys have the pattern 'X_Y', where X is the numerical row index and Y is the numerical column index, both starting from zero.",
+                        "additionalProperties": {
+                            "$ref": "#/components/schemas/TableExtendedData"
+                        }
+                    },
                     "metadata": {
                         "$ref": "#/components/schemas/RenderContentMetadata"
                     }
                 }
             },
+            "TableExtendedData": {
+                "type": "object",
+                "properties": {
+                    "colspan": {
+                        "type": "integer"
+                    },
+                    "rowspan": {
+                        "type": "integer"
+                    }
+                }
+            },
             "Step": {
                 "required": [
                     "type",