[Docs] Add SQL Compatibility reference

Add a user-facing SQL compatibility reference at
docs/user/general/sql-compatibility.rst, modeled on MySQL's Standards
Compliance and PostgreSQL's SQL Conformance pages.

The new page covers:

* SQL standards compliance (ANSI SQL-92 plus selected features from
  SQL:1999, SQL:2003, SQL:2008, and SQL:2016).
* Extensions to standard SQL that the new engine accepts (hyphenated
  and backtick-quoted identifiers, GROUP BY by alias or ordinal,
  LIMIT N, optional FROM, relevance search functions).
* Deliberate deviations from standard SQL, each driven by backward
  compatibility with OpenSearch SQL V2 (double-quoted strings,
  GROUP BY alias resolution, lenient type coercion, backslash
  escapes, MATCH as a non-reserved identifier).
* A 'Migrating from SQL V2' chapter enumerating constructs that V2
  accepted but the new engine rejects: lexical and literal
  differences, identifier differences, type-system differences, an
  extensive function-differences catalogue (~90 V2 functions grouped
  by purpose with standard SQL equivalents where one exists), clause
  and syntactic differences, metadata-statement differences, and
  output-format differences.

Wire the new page into the Sphinx TOC under Language Structure.

Signed-off-by: Chen Dai <daichen@amazon.com>

Author: dai-chen