docs: Missing SHA256 checksums for Windows MSI packages

## Bug Report

**Describe the bug**
The official Fluent Bit documentation lists SHA256 hashes for .exe installers and .zip archives, but the cryptographic hashes for the distributed .msi Windows installer packages are entirely missing from the documentation tables and the download directory.

**To Reproduce**
1. Go to the official Fluent Bit Windows installation manual: https://docs.fluentbit.io/manual/installation/downloads/windows
2. Scroll to the package tables or direct links.
3. Observe that there are no SHA256 checksum listings or corresponding verification links for any .msi assets (e.g., for version v5.1.1).

**Expected behavior**
Every released binary type, including the .msi packages, should have its official SHA256 hash published transparently in the documentation or attached as a SHA256SUMS manifest in the distribution channels.

**Your Environment**
* Version used: v5.1.1-win64 (and prior versions using the MSI format)
* Configuration: N/A (Installation/Compliance blocker)
* Server type and version: Windows Server e.g. v2019, v2022, v2025
* Filters and plugins: None

**Additional context**
In enterprise environments, corporate IT security policies and strict compliance regulations strictly prohibit the deployment of software binaries that cannot be verified against an official, vendor-published cryptographic hash. Because the .msi hashes are missing from the official sources, we are currently blocked from deploying Fluent Bit across our Windows infrastructure via standard automation.

Start with the Fluent Bit Windows installation manual at the linked downloads page and inspect its package tables, direct links, and the distribution directory for the v5.1.1 MSI assets. Done means every released MSI has an official SHA256 hash published in the documentation or an attached SHA256SUMS manifest, with corresponding verification links where applicable.

Written by the indexing model from the issue text.