Restructure SQL docs: add function coverage table, fix errors, consol…

[christophdb]

Overview of changes:

• Add function support overview table to functions.md with all tested functions and their MySQL equivalents (tested against SeaTable 6.1.8)
• Fix substract() → subtract() typo in functions.md
• Mark hours() and isoweeknum() as not supported in SQL queries
• Document that constants (pi, e) only work inside function calls
• Fix incorrect ORDER BY rule (column does NOT need to be in SELECT)

Structural improvements:

• Merge clauses.md into select.md (WHERE, GROUP BY, HAVING, ORDER BY)
• Move list operators (HAS ANY OF, etc.) from extended-syntax.md to select.md
• Move formulas-in-SQL notes and Big Data indexes to index.md
• Add quick reference tables (supported/not supported) to index.md
• Remove clauses.md and extended-syntax.md, add redirects
• Update mkdocs.yml navigation

New structure:
SQL/
├── index.md — Intro + Quick Reference + Formulas + Big Data
├── select.md — SELECT with all clauses, operators, JOINs
├── insert-update-delete.md (unchanged)
├── data-types.md (unchanged)
└── functions.md — Coverage table + complete function reference