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: 840f3b8c-7f95-4189-8ccd-5c0a940f9986

📥 Commits

Reviewing files that changed from the base of the PR and between 38e86b5 and d6220f7.

📒 Files selected for processing (1)

• docs/source/installation.md

✅ Files skipped from review due to trivial changes (1)

• docs/source/installation.md

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📝 Walkthrough

Walkthrough

Installation documentation now includes Windows-specific instructions for setting BUILD_MONAI=1 before GitHub-based and editable pip install commands. Both flows now show separate cmd.exe and PowerShell examples.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	Clear and specific; it states the docs now include Windows BUILD_MONAI install instructions.
Description check	✅ Passed	Matches the template sections and includes description, type of changes, and testing notes.
Linked Issues check	✅ Passed	The docs now explain Windows cmd.exe/PowerShell setup for BUILD_MONAI before pip install, satisfying #6119.
Out of Scope Changes check	✅ Passed	Changes stay confined to installation docs and directly support the Windows BUILD_MONAI instructions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[dzenanz]

Thanks for updating the docs. Sometimes it is obvious what needs to be done, but it is better to have the correct thing in the docs.

[Shizoqua]

Thanks for the review, @dzenanz! Is there anything else needed on my end before this can be merged, or is it just waiting on a second approval? Happy to make any adjustments.

[dzenanz]

I think it is just waiting for an approval by someone with write permission.

[Shizoqua]

Rebased onto latest upstream/dev. Still ready — just waiting on a maintainer with write permissions to merge this one (already has @dzenanz's approval).