[fix](doc) add GROUP_CONCAT ORDER BY examples to 2.1/3.x group-concat docs

[boluor]

Summary

The GROUP_CONCAT page syntax already advertises [ORDER BY {<col_name>|<expr>} [ASC|DESC]], but the Examples section on 2.1/3.x only showed the basic and DISTINCT forms, so a reader couldn't see how ORDER BY actually attaches inside GROUP_CONCAT. The current/4.x pages already carry two ORDER BY examples — backport them to 2.1/3.x (EN + zh), with a version note on the DISTINCT + ORDER BY combination so users on older patch releases don't hit a parser error.

Added examples (matching the form already shipped in current/4.x):

select GROUP_CONCAT(value ORDER BY value DESC) from test;
-- => "c, c, b, a"

-- DISTINCT + ORDER BY is supported since 2.1.6 / 3.0.2 — see note above the example
select GROUP_CONCAT(DISTINCT value ORDER BY value DESC) from test;
-- => "c, b, a"

Version verification (from Doris source)

• Basic GROUP_CONCAT(... ORDER BY ...): introduced in Nereids by [feature](Nereids) Support lots of aggregate functions doris#15671 + [fix](Nereids) wrong result of group_concat with order by or null args doris#16081 (Jan 2023, before 2.0 GA) → works on 2.1.x and 3.x.
• GROUP_CONCAT(DISTINCT ... ORDER BY ...): [fix](nereids)support group_concat with distinct and order by doris#38080, labels dev/2.1.6-merged, dev/3.0.2-merged → only valid on 2.1.6+ and 3.0.2+. The page therefore carries an inline :::note flagging this minimum-version requirement (2.1.6 in the 2.1 docs, 3.0.2 in the 3.x docs).

Test plan

• All 4 backported files now have 6 GROUP_CONCAT(...) example calls, matching current/4.x.
• DISTINCT + ORDER BY example is accompanied by a minimum-version note (2.1.6 for the 2.1 docs, 3.0.2 for the 3.x docs).
• Dead-link check passes.
• CI build.

Closes #3487