Update development env docs for Windows

[morotsgurka]

I noticed that the development setup for esphome has Windows separate at the bottom with different instructions and I'm not sure why?
And when actually reading the script/setup.bat file it even prints out the correct command to activate the virtual environment.

• I cloned the repo.
• I ran scripts/setup
• I ran venv/Scripts/activate
• Compile a sample config file with no issues

Also. The README contribute section does not mention to install the required pip packages (like mkdocs).

What changed?

• Added Windows section to the venv activate section
• Added pip install -r requirements.txt step before serve command in README

WIP
This probably needs more work since having both the separate instructions (at the bottom) and the working windows venv at the top could be confusing. At least clarify the reason for doing so?

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[Copilot]

Pull Request Overview

Updates development environment documentation to improve cross-platform setup instructions and adds missing dependency installation step for contributors.

• Consolidates Windows activation instructions with Linux/macOS instead of keeping them separate
• Adds missing pip requirements installation step to README contribution guide

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/contributing/development-environment.md	Adds Windows PowerShell activation command alongside existing Linux/macOS instructions
README.md	Adds missing pip install requirements step to contribution workflow

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