feat(sql-joins): add JoinsInSQL folder with join concepts

What
- Introduced `JoinsInSQL` folder to organize SQL join concepts and examples.
- Documented key join types:
  - INNER JOIN → returns only matching rows between tables.
  - LEFT JOIN → returns all rows from left table + matching rows from right.
  - RIGHT JOIN → returns all rows from right table + matching rows from left.
  - FULL OUTER JOIN → returns all rows when there’s a match in either table.
  - CROSS JOIN → produces Cartesian Product of rows.
- Explained syntax and usage for each join type.
- Added illustrative scenarios with example tables (`Employees`, `Departments`).

Why
- Joins are central to relational database operations.
- Developers often confuse different join types; this provides clear guidance.
- Organizing content into a dedicated folder improves readability and reusability.

How to use
- Refer to individual files or notes inside `JoinsInSQL` for examples.
- Apply appropriate join type depending on use case:
  - INNER JOIN → strict matches.
  - LEFT/RIGHT JOIN → preserve one table’s rows.
  - FULL JOIN → combine both sides even if no match.
  - CROSS JOIN → generate all combinations.

Real-life Applications
- HR systems (employees × departments).
- E-commerce (orders × customers).
- Finance (transactions × accounts).
- Reporting dashboards combining multiple data sources.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan