github: enhance pull request template (port from master)

[mr-raj12]

Description

Port the enhanced PR template from master (#9335) to 1.4-maint, as requested by @ThomasWaldmann in
#9335 (comment).

Changes (identical to #9335):

• Replace minimal plain-text template with a structured Markdown version
• Add ## Description and ## Checklist sections
• Move instructions into HTML comments
• Rename to .md for consistency with ISSUE_TEMPLATE.md

Checklist

• PR is against 1.4-maint
• New code has tests and docs where appropriate — not applicable, template-only change
• Tests pass — not applicable, no code changes
• Commit messages are clean and reference related issues

[ThomasWaldmann]

Thanks!