Docs: document supported Python version for backend

[riyagoyal01]

Fixes #1055

###What this PR does

• Adds a “Supported Python Versions” section to README.md
• Adds a reference in CONTRIBUTING.md to clarify backend Python compatibility
• Focuses on helping new contributors avoid installation errors, especially on Windows
• Makes Python version requirements explicit (3.11 supported, others may fail)

###Context

New contributors may use unsupported Python versions (e.g., 3.9 or 3.13+) when setting up the backend.
CI currently runs backend and sync-microservice builds using Python 3.11, but this was not documented.
On Windows, using unsupported Python versions often leads to dependency installation failures or confusing errors.

###Solution

This PR updates documentation:

• README.md – Adds a clear section “Supported Python Versions” with compatibility notes
• CONTRIBUTING.md – Adds a note pointing to the supported backend Python versions

###Tested

• Verified README.md displays the supported Python version section correctly
• Verified CONTRIBUTING.md references the backend Python version
• Checked formatting and links in MkDocs docs website setup