docs(sql): add TRUNCATE syntax.

[ashigeru]

This pull request updates the SQL feature documentation to add support for the TRUNCATE TABLE statement. The changes clarify planned features, syntax, and reserved words to reflect the addition of this statement.

Feature addition:

• Added TRUNCATE TABLE to the list of planned SQL features and feature syntax examples in docs/sql-features.md. [1] [2]
• Documented the syntax and options for TRUNCATE TABLE, including RESTART IDENTITY and CONTINUE IDENTITY options.

Reserved words update:

• Added TRUNCATE to the list of reserved SQL keywords under the "T" section in docs/sql-features.md.

[ashigeru]

related to: https://github.com/project-tsurugi/tsurugi-issues/issues/812