fix: update Markdown cheatsheet links in issue templates 1, 2, 3, and 4

[menschenkind]

Summary

This PR updates the outdated Markdown cheatsheet PDF links in issue templates 1, 2, 3, and 4.

Changes

• Replaced outdated link with the correct Markdown cheatsheet PDF: https://enterprise.github.com/downloads/en/markdown-cheatsheet.pdf

Motivation

The previous link pointed to an outdated resource. Updating ensures contributors have access to the correct Markdown reference. The linked PDF is more useful than GitHub’s standard Markdown documentation, especially for these tasks, as it explains the different header levels (H1, H2, etc.).

Related Issue

Fixes #34