[fix](doc) median.md: add missing setup in EN; align Example 2 SQL with its result

[boluor]

Summary

Two fixes to sql-manual/sql-functions/aggregate-functions/median.md, both found by running every example on a real Doris 4.1.1 cluster.

Fix 1 — EN page is missing the setup block

The EN page jumps straight to SELECT … FROM log_statis … without first creating the table or inserting fixture rows, so every example fails with Table [log_statis] does not exist if a reader copy-pastes them. The ZH version already has the setup block. This PR brings the EN page in line — same CREATE TABLE + INSERT content, no other changes.

Fix 2 — Example 2 SQL doesn't match its result block (EN + ZH)

The second example on both pages had:

• SQL block: SELECT median(scan_rows) FROM log_statis GROUP BY datetime;
• Result block: first line was the literal select median(scan_rows) from log_statis where scan_rows is null; (a captured command echo from mysql CLI), followed by a single NULL row.

So the result is actually the output of WHERE scan_rows IS NULL, not GROUP BY datetime — the SQL block was pasted from a different query by mistake.

Aligned the SQL block to match the result (the WHERE form). Two reasons that direction over the other:

1. The result body (single NULL row) is what WHERE scan_rows IS NULL returns; the GROUP BY datetime form returns two median rows on the fixture data.
2. Demonstrates the Description's note that median returns NULL when the group has no valid data.

A reader can now copy-paste Example 2 and reproduce the documented output.

Verification on Apache Doris 4.1.1

With the setup block applied:

-- Example 1
SELECT datetime, median(scan_rows) FROM log_statis GROUP BY datetime;
-- → (2025-08-25 10:00 → 50.0), (2025-08-25 11:00 → 30.0)   ✓

-- Example 2 (after this fix)
SELECT median(scan_rows) FROM log_statis WHERE scan_rows IS NULL;
-- → NULL   ✓

Changes

• versioned_docs/version-4.x/sql-manual/sql-functions/aggregate-functions/median.md — add setup block (+16); fix Example 2 SQL (1 line)
• i18n/zh-CN/docusaurus-plugin-content-docs/version-4.x/sql-manual/sql-functions/aggregate-functions/median.md — fix Example 2 SQL (1 line)

Total: 2 files, +17 / -1 lines, split into 2 logically-distinct commits for review.

Detection

Discovered by running median.md examples on a single-BE Apache Doris 4.1.1 cluster as part of a broader docs-vs-implementation verification effort. The same effort already produced doris-website#3781 (merged) and doris-website#3786 (open).