Docs: clarify upstream remote setup and fork sync steps for contributors (#3236)

* docs: clarify upstream remote setup and fork sync steps

* docs: refine fork sync section structure

* docs: apply review feedback to fork sync section

* Update CONTRIBUTING.md

---------

Co-authored-by: Arkadii Yakovets <arkadii.yakovets@owasp.org>
Co-authored-by: Arkadii Yakovets <2201626+arkid15r@users.noreply.github.com>

Author: SuyashJain17

CONTRIBUTING.md

@@ -492,9 +492,40 @@ flowchart TD
     linkStyle 24 stroke:#f44336,stroke-width:2px
 ```
 
+### Keep Your Fork in Sync with Upstream
+
+To avoid working on an outdated copy of Nest (and to reduce merge conflicts), contributors may find it helpful to keep their fork synchronized with the main OWASP Nest repository.
+
+<details>
+<summary>Setting up the upstream remote</summary>
+
+If you haven't added the upstream remote yet, add it using:
+
+```bash
+git remote add upstream https://github.com/OWASP/Nest.git
+```
+
+Verify that the upstream remote has been added by running:
+
+```bash
+git remote -v
+```
+
+This should show both `origin` (your fork) and `upstream` (the main repository) remotes.
+
+</details>
+
+Before working on a **new** feature or issue, update your local `main` branch from `upstream/main`:
+
+```bash
+git checkout main
+git fetch upstream
+git merge upstream/main
+```
+
 ### 1. Find Something to Work On
 
-- Check the **Issues** tab for open issues: [https://github.com/owasp/nest/issues](https://github.com/owasp/nest/issues)
+- Check the **Issues** tab for open issues: [https://github.com/OWASP/Nest/issues](https://github.com/OWASP/Nest/issues)
 - Found a bug or have a feature request? Open a new issue.
 - Want to work on an existing issue? Ask the maintainers to assign it to you before submitting a pull request.
 - New to the project? Start with issues labeled `good first issue` for an easier onboarding experience.