docs: Fix incorrect directory path in Quick Start and update environment/dependency info

[Drishti160803]

Hi Mentors,
While setting up the project locally, I noticed a few inconsistencies in the README.md that could hinder the onboarding process for new contributors. I would like to fix these as part of "documentation fixation."
-Proposed Changes:

1. Directory Path Bug: In the "Using Docker" section, the git clone command creates a folder named VCell-AI, but the subsequent cd command points to VCell-GSoC. This causes a "directory not found" error. I will update it to cd VCell-AI.
2. Node.js Version: Updated the prerequisite from Node.js 18+ to Node.js 20+. Since the project uses Next.js 15, version 20 (LTS) is the recommended stable environment.
   -Typo Fixes:
3. Corrected "Markitdown" to MarkItDown (matching the official Microsoft library naming).
4. Standardized capitalization of Biomodel across the document.
   -Readability: Minor phrasing improvements in the "LLM calls" and "Architecture" sections for better clarity.

I have already identified these fixes and would like to submit a Pull Request. Could you please assign this issue to me?
Thanks!