Fix broken links in contributing page

[nixphix]

All of the relative links in the contributing page are broken, I see the relative linked docs are not copied from the repo.

Following are the broken links

• [x] https://prometheus-operator.dev/docs/prologue/contributing/LICENSE
• [x] https://prometheus-operator.dev/docs/prologue/contributing/code-of-conduct.md
• [x] https://prometheus-operator.dev/docs/prologue/contributing/DCO
• [x] https://prometheus-operator.dev/docs/prologue/contributing/README.md
• [x] https://prometheus-operator.dev/docs/prologue/contributing/TESTING.md
• [ ] https://prometheus-operator.dev/docs/prologue/contributing/scripts/run-external.sh

Fixing scripts/run-external.sh is tricky, to render it properly with Hugo I need to add a layout shortcode to wrap the script within a code block.