docs: add REVIEW_GUIDELINES.md

[UtkarshAnandd]

Closes discussion #6925

Compiling the Review Guide discussed in the Ideas thread into a
REVIEW_GUIDELINES.md file, incorporating suggestions from:

• @walterbender
• @vyagh
• @farhan-momin
• @karthik-dev56
• @zealot-zew

The guide covers:

• Pre-PR checklist for contributors
• Best practices for reviewers
• Contributor journey stages

[github-actions]

🧪 Jest Test Results

✅ All Jest tests passed! This PR is ready to merge.

Coverage: Statements: 48.37% | Branches: 40.07% | Functions: 53.06% | Lines: 48.76%
Master Coverage: Statements: 48.37% | Branches: 40.07% | Functions: 53.06% | Lines: 48.76%

[Ashutoshx7]

@vyagh @zealot-zew guys your thoughts on this

[vyagh]

Out of scope for review guidelines, right?

[vyagh]

1. browser smoke test (when applicable) is missing, that's a very important one.
2. We should explicitly mention the problem where contributors open a single PR, which clearly should have been multiple PRs (this is quite common)
3. would be nice to mention about "Thoroughly check the folder directory especially for tests" comment from the discussion, sometimes new test files are created when the tests could've been just added to existing test files.
4. let's not close the discussion even after merging this PR, I believe the discussion can be used further to keep improving this guide?

[ssz2605]

Few more points that could be added under the contributor checklist:

• For feature, UI/UX, or workflow changes, include before-and-after screenshots, GIFs, or a short video demonstrating the change.
• For performance-related changes, provide before-and-after benchmark results, performance graphs, or screenshots of relevant metrics along with a brief explanation of the measurable impact created by the optimization.

This helps reviewers understand the change more quickly, validate the claimed improvements, and reduces the effort required to reproduce the results locally.