Add Examples and usecase for Lag windows function

[ultralegendary]

Description

I have added examples for the lag function and use case for them under windows functions for SQL. Edited file - content/sql/concepts/window-functions/terms/lag/lag.md Please review and let me know if any changes required.

Issue Solved

5395

Type of Change

• Editing an existing entry (adding examples and single line usecases)

Checklist

• [x] All writings are my own.
• [x] My entry follows the Codecademy Docs style guide.
• [x] My changes generate no new warnings.
• [x] I have performed a self-review of my own writing and code.
• [x] I have checked my entry and corrected any misspellings.
• [x] I have made corresponding changes to the documentation if needed.
• [x] I have confirmed my changes are not being pushed from my forked main branch.
• [x] I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• [x] I have linked any issues that are relevant to this PR in the Issues Solved section.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
3 out of 4 committers have signed the CLA.

:white_check_mark: cigar-galaxy82
:white_check_mark: PragatiVerma18
:white_check_mark: Radhika-okhade
:x: Dhivakaran V R

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[ultralegendary]

hi @cigar-galaxy82 , I have signed the Contributor License Agreement. Please let me know if I have missed anything else.

[Radhika-okhade]

Hey! @ultralegendary. Could you please sign the CLA so that we can proceed with the second review?