docs: add Windows BUILD_MONAI install instructions

[Shizoqua]

Fixes #6119 .

Description

docs/source/installation.md only documents the POSIX inline form BUILD_MONAI=1 pip install ... for building the MONAI C++/CUDA extensions. This VAR=value command syntax is not supported by Windows cmd.exe or PowerShell, so the documented commands fail out of the box on Windows.

This adds short cmd.exe and PowerShell snippets to both install flows (Option 1 system-wide and Option 2 editable): set the environment variable first, then run the existing pip install command. The set BUILD_MONAI=1 form is the one confirmed working on Windows 11 in the issue thread.

Types of changes

• Non-breaking change (documentation only).
• Documentation updated.

How tested

• Verified the new bat/powershell fenced blocks use valid Pygments lexers and lex without error tokens, so the strict docs build (build_docs.yml) emits no new warnings.
• pre-commit markdown hooks (end-of-file, trailing-whitespace, mixed-line-ending) pass on the changed file.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 767de70e-8a50-4e00-84b8-d0c482a2e1b8

📥 Commits

Reviewing files that changed from the base of the PR and between eccefc5 and 38e86b5.

📒 Files selected for processing (1)

• docs/source/installation.md

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📝 Walkthrough

Walkthrough

Installation documentation now includes Windows-specific instructions for setting the BUILD_MONAI=1 environment variable before running GitHub-based pip install commands. Two installation flows are covered: standard installation and editable installation. Each flow includes separate cmd.exe and PowerShell code block examples showing the correct syntax for Windows environments.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding Windows-specific BUILD_MONAI installation instructions to documentation.
Description check	✅ Passed	The description is complete with fix reference, clear explanation of the problem, solution, change types, and testing verification.
Linked Issues check	✅ Passed	The PR addresses issue #6119 by adding Windows cmd.exe and PowerShell code blocks showing the correct syntax for setting BUILD_MONAI=1 before pip install commands.
Out of Scope Changes check	✅ Passed	All changes are limited to documentation updates in installation.md with Windows-specific build instructions, directly addressing the linked issue.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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