chore(contributions): update local environment setup documentation (#41401)

Updated developer documentation to standardize Node.js version
requirement to 20.11.1 and added comprehensive RTS (Real-Time Server)
setup instructions in ServerSetup.md for both MacOS/Linux and Windows
WSL2 environments.

The changes clarify when RTS is needed (for collaborative features,
real-time sync, WebSocket communication) versus optional (for basic
backend changes), include step-by-step setup instructions with proper
version management using nvm/fnm, and position RTS startup before the
Java server to reflect the correct initialization order. Also updated
environment configuration examples and git root storage folder setup
documentation.

## Description
> [!TIP]  
> _Add a TL;DR when the description is longer than 500 words or
extremely technical (helps the content, marketing, and DevRel team)._
>
> _Please also include relevant motivation and context. List any
dependencies that are required for this change. Add links to Notion,
Figma or any other documents that might be relevant to the PR._


Fixes #`Issue Number`  
_or_  
Fixes `Issue URL`
> [!WARNING]  
> _If no issue exists, please create an issue first, and check with the
maintainers if the issue is valid._

## Automation

/ok-to-test tags="@tag.All"

### :mag: Cypress test results
<!-- This is an auto-generated comment: Cypress test results  -->
> [!TIP]
> 🟢 🟢 🟢 All cypress tests have passed! 🎉 🎉 🎉
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/20489814780>
> Commit: 16ee466d2b8465e471aa72db035be8e115dda83f
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=20489814780&attempt=2"
target="_blank">Cypress dashboard</a>.
> Tags: `@tag.All`
> Spec:
> <hr>Wed, 24 Dec 2025 22:46:55 UTC
<!-- end of auto-generated comment: Cypress test results  -->


## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [x] No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
  * Updated Node.js requirement to 20.11.1 for setup.
* Added comprehensive Real-Time Server (RTS) setup guidance, including
RTS-first startup and revised step ordering.
* Expanded and reorganized setup instructions for Windows, WSL, macOS,
and Linux.
  * Minor formatting tweaks to prompts in setup docs.

* **Chores**
* Added an option to configure a custom Git repository root for local
development.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->