docs: add docs for 'to_decimal' (#2450)

Co-authored-by: z <[email protected]>

Author: KKould

docs/en/sql-reference/20-sql-functions/02-conversion-functions/to-decimal.md

@@ -0,0 +1,69 @@
+---
+title: TO_DECIMAL
+---
+
+Converts a value to DECIMAL data type.
+
+## Syntax
+
+```sql
+TO_DECIMAL
+( <expr> [, '<format>' ] [, <precision> [, <scale> ] ] )
+```
+
+## Arguments
+
+| Arguments | Description                                                                                                                                                                             |
+|-----------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| expr      | An expression of a boolean, number, string, or variant type.                                                                                                                            |
+| format    | Optional. A format string specifying the desired precision and scale for the decimal result. It may define the total number of digits and the number of digits after the decimal point. |
+| precision | Optional. Total number of significant digits in the result, the default is 38.                                                                                                          |
+| scale     | Optional. Number of digits after the decimal point. Must not exceed precision, the default is 0.                                                                                        |
+
+### Format Specifiers
+
+| Format Specifier   | Description                                                                     | Example Input |
+|--------------------|---------------------------------------------------------------------------------|---------------|
+| '9'                | Optional digit (accepts digit or nothing, but this implementation requires one) | "123"         |
+| '0'                | Required digit (same as 9 in current implementation)                            | "123"         |
+| 'D'                | Decimal point (accepts '.' or ',', always normalized to '.')                    | "12.34"       |
+| 'G'                | Group (thousands) separator (expects ',' in input)                              | "1,234"       |
+| 'S'                | Sign ('+' or '-', applied as prefix to result)                                  | "-123"        |
+| Literal character	 | Any literal character must appear in the input exactly                          | "NT$123"      |
+
+## Examples
+
+```sql
+SELECT TO_DECIMAL('1234.56');
+
+╭───────────────────────╮
+│ to_decimal('1234.56') │
+├───────────────────────┤
+│                  1235 │
+╰───────────────────────╯
+
+SELECT TO_DECIMAL('1234.56', '9999D99');
+
+╭──────────────────────────────────╮
+│ to_decimal('1234.56', '9999D99') │
+├──────────────────────────────────┤
+│                             1235 │
+╰──────────────────────────────────╯
+
+SELECT TO_DECIMAL('1234.56', 38, 1);
+
+╭──────────────────────────────╮
+│ to_decimal('1234.56', 38, 1) │
+├──────────────────────────────┤
+│                       1234.6 │
+╰──────────────────────────────╯
+
+SELECT TO_DECIMAL('NT$1234.56', 'NT$9999D99', 38, 1);
+
+╭───────────────────────────────────────────────╮
+│ to_decimal('NT$1234.56', 'NT$9999D99', 38, 1) │
+├───────────────────────────────────────────────┤
+│                                        1234.6 │
+╰───────────────────────────────────────────────╯
+
+```