sql: document ALTER TABLE, TRUNCATE TABLE

Summary

• sql: document ALTER TABLE, TRUNCATE TABLE

https://docs.binary.picodata.io/picodata/docs_alter_table/reference/sql/alter_table/

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• Close #docs488, docs484, #1752 (closed), #1753 (closed)
  • Cherry-pick to: none / 25.1 / 24.6
  • Docs follow-up: not necessary / new issue / docs updated in this MR
• Changelog is updated