[doc] STRUCT_ELEMENT example in 3.x/2.1: undefined table + outdated struct display format

### Page

`STRUCT_ELEMENT` function reference — `version-3.x` and `version-2.1`, EN + ZH:
- `versioned_docs/version-3.x/sql-manual/sql-functions/scalar-functions/struct-functions/struct-element.md`
- `versioned_docs/version-2.1/.../struct-element.md` (+ the two `i18n/zh-CN` copies)

### Two problems with the second example

```sql
select struct_col, struct_element(struct_col, 'f1') from test_struct;
```

**1. `test_struct` is never created.** The page has no `CREATE TABLE`/`INSERT` for `test_struct`, so copy-pasting the example fails with `table does not exist`.

**2. The printed output uses an outdated STRUCT display format.** The doc shows structs rendered positionally:

```
| struct_col | ... |
| {1, 2, 3, 4, 5} | 1 |
| NULL | NULL|
| {1, NULL, 3, NULL, 5} | 1 |
```

But current Doris (verified on **3.1.4** and **2.1.11**) renders STRUCT columns as JSON with field names:

```
| {"f1":1, "f2":2, "f3":3, "f4":4, "f5":5} | 1 |
| {"f1":1, "f2":null, "f3":3, "f4":null, "f5":5} | 1 |
```

So even after defining `test_struct`, the `struct_col` column in the documented output is unreproducible as written — the example is stale relative to the actual struct rendering on these release lines.

### Suggested fix

Add a `CREATE TABLE test_struct (...) / INSERT` setup block **and** update the example's expected output to the current JSON struct rendering (`{"f1":1, ...}`, nested `null` lowercase). I verified that a 5-row `struct` table reproducing the row values (`{1,2,3,4,5}`, `{1,1000,10000000,100000000000,100000000000}`, `{5,4,3,2,1}`, a `NULL` row, and `{1,null,3,null,5}`) yields `struct_element(struct_col,'f1')` = 1, 1, 5, NULL, 1 exactly — only the raw `struct_col` rendering needs the format update.

I'm intentionally leaving this out of the phantom-table-setup PR series because it's more than a missing-table fix — it requires changing the documented output to match current struct rendering, which is an editorial call on these older release lines.