Update pull request template with authorship confirmation

Added a confirmation checkbox for authorship and licensing.

Author: Perksey

.github/pull_request_template.md

@@ -1,6 +1,8 @@
 # Summary of the PR
 A short summary of this PR and what it adds/removes/fixes.
 
+- [ ] I confirm that I am the author of this code and release it to the Silk.NET project under the MIT license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").
+
 # Related issues, Discord discussions, or proposals
 Links go here.