docs: seed the SQL reference from DataFusion 54, and cross-link it

The function catalogue is now real rather than a pointer. Scalar functions are
listed by taxonomy -- math, string, unicode, regex, date/time, array, map,
struct, crypto, type -- and the names come from what datafusion-functions 54
actually registers, not from what DataFusion documents on main. Aggregate and
window pages gain the same treatment.

Two things the audit corrected rather than copied.

SHOW TABLES does not work. DataFusion implements it over information_schema,
which this session does not enable, so it fails at plan time. It was documented
as working here, on the SHOW page and in three Manage tables; all four are
fixed, and the SQL overview now lists the absence alongside SHOW COLUMNS.

Prepared statements are a Flight SQL protocol feature, not a PREPARE statement.
A script carrying PREPARE ... AS SELECT will not run, while the same intent
expressed through a client will. The new page says so rather than copying
DataFusion's syntax.

Every section of the Documentation tab that has a SQL counterpart now links to
it -- 27 of 76 non-SQL pages, each pointing at the specific statement rather
than at the reference index.

docs/web/sql/select.mdx

@@ -39,7 +39,7 @@ Projection pushes into the scan in all three -- an unselected column is never
 read off storage. On a table of images or video that is the difference between a
 fast query and a slow one, so **prefer naming columns over `SELECT *`**.
 
-Row metadata is available as [pseudo-columns](/sql/functions#pseudo-columns)
+Row metadata is available as [pseudo-columns](/sql/data-types#pseudo-columns)
 (`_rowid`, `_rowaddr`, `_rowoffset`) but only when named, so they stay out of
 `SELECT *`.