Add DB indexes document

[shorodilov]

Summary

Database indexes should be explained. What is the main purpose?

References

• https://www.freecodecamp.org/news/database-indexing-at-a-glance-bb50809d48bd/
• https://www.codecademy.com/article/sql-indexes
• https://medium.com/@aditya.barik32/database-indexing-secrets-d1f93e67bb1b