docs: route main documentation to the Windows release branch #4756

User Story

As the maintainer preparing OpenShell's native Windows release, I need documentation on main to direct Windows users to the windows release branch while keeping the Windows implementation on that branch.

Problem Statement

Main's top-level documentation lacks consistent routing to the maintained native Windows implementation. Merging the entire Windows branch to obtain those docs would also bring implementation changes into main.

Impact / Why This Matters

Readers can mistake main's installer, release artifacts, or older MXC snapshot for the Windows release contract. Manually discovering the correct branch is an insufficient workaround for users following the primary docs.

Description

Publish a documentation-only update based on main: add the Windows guide, update root technical docs and published platform entry points, and link Windows runtime/configuration details explicitly to windows. Preserve main's current code and unrelated documentation changes.

Context

A companion PR targeting windows will add the canonical Windows guide. Merge that PR first so branch-qualified links to the new guide resolve. This task is separate from the implementation-detail corrections in #4645 and PR #4646.

Acceptance Criteria

• Main's docs identify windows as the native Windows implementation and release branch.
• Windows-only source and configuration links explicitly use windows.
• Older MXC documentation on main is distinguished from the current Windows release contract.
• The PR changes documentation only, with no Windows runtime or build implementation changes.
• Applicable Markdown, navigation, link, and documentation checks are reported.