docs: add Windows setup instructions for local docs build

[NikolaiKryshnev]

Summary

While setting up the Solidity documentation on Windows, I found that the existing instructions only covered macOS/Linux (./docs.sh, python3 -m http.server -d ...).
This PR adds a dedicated Windows (PowerShell) guide.

Changes

• Added step-by-step Windows instructions for creating a virtual environment, installing dependencies, and building docs.
• Provided correct http.server command syntax for Windows (--directory instead of -d).

Motivation

This update ensures that contributors on Windows can follow a clear and working process to build and serve the docs locally without having to troubleshoot missing dependencies or command syntax differences.

[github-actions]

Thank you for your contribution to the Solidity compiler! A team member will follow up shortly.

If you haven't read our contributing guidelines and our review checklist before, please do it now, this makes the reviewing process and accepting your contribution smoother.

If you have any questions or need our help, feel free to post them in the PR or talk to us directly on the #solidity-dev channel on Matrix.