installation: downloads: windows: page still documents 5.1.1 after the v5.1.2 release

## Summary

`installation/downloads/windows.md` still documents 5.1.1 as the latest stable release, but v5.1.2 has been released. The page can't be updated yet because the 5.1.2 Windows artifacts aren't published to `packages.fluentbit.io`.

## Current state

`installation/downloads/windows.md` is pinned to 5.1.1 in several places:

- Line 116: `The latest stable version is 5.1.1.`
- Lines 122-127: the installer table, with download links and SHA256 checksums for `win32`, `win64`, and `winarm64` (`.exe` and `.zip`)
- Lines 135-137: the MSI installer links for `win32`, `win64`, and `winarm64`
- Lines 142 and 152: the `Get-FileHash` and `Expand-Archive` examples

## Why this is blocked

Updating the page needs the six SHA256 checksums for the 5.1.2 installers. Those files return `NoSuchKey`:

```console
$ curl -s https://packages.fluentbit.io/windows/fluent-bit-5.1.2-win64.exe.sha256

NoSuchKeyThe specified key does not exist....
```

All six return the same error:

- `fluent-bit-5.1.2-win32.exe.sha256`
- `fluent-bit-5.1.2-win32.zip.sha256`
- `fluent-bit-5.1.2-win64.exe.sha256`
- `fluent-bit-5.1.2-win64.zip.sha256`
- `fluent-bit-5.1.2-winarm64.exe.sha256`
- `fluent-bit-5.1.2-winarm64.zip.sha256`

The URL pattern is correct. The equivalent 5.1.1 file resolves and matches what the page already documents:

```console
$ curl -s https://packages.fluentbit.io/windows/fluent-bit-5.1.1-win64.exe.sha256
877771b17776cebc066e68bbde9bf5c85c98814c5dd19ceef45a4561d1bae81c fluent-bit-5.1.1-win64.exe
```

The `v5.1.2` tag was released in `fluent/fluent-bit` on 2026-09-05, so the release exists but the Windows packages behind it don't appear to have been published yet.

## What's needed

1. Publish the 5.1.2 Windows installers and their `.sha256` files to `packages.fluentbit.io/windows/`.
2. Run `update-release-version-docs.sh` with the `WIN_*_HASH` values for the new installers, which rewrites the "latest stable version" line, the installer filename patterns, and the checksums.

## Related

- `installation/downloads/docker.md` has a separate PR adding the 5.1.2 container image tags. The Docker Hub images for `5.1.2` and `5.1.2-debug` are published, so that half isn't blocked.
- `update-release-version-docs.sh` only updates `docker.md` and `windows.md`. Per-plugin "available in vX.Y" callouts elsewhere in the docs are maintained separately.