Add essential sections to MySQL: Alter, Indexes, and Transactions

I have enhanced the MySQL cheat sheet by adding three essential sections that were previously missing:

1. Altering Table Structure: Commands for adding, modifying, renaming, and dropping columns/tables.
2. Indexes (Performance Tuning): Added CREATE INDEX, SHOW INDEX, and the EXPLAIN command for query analysis.
3. Transactions (Data Integrity): Added START TRANSACTION, COMMIT, and ROLLBACK for managing database consistency.