Update issue templates

[hillaryj]

🗣 Description

Moves the instructions for completing issue templates into comments.

💭 Motivation and context

Makes it easier to fill out by having instructions remain invisible.

🧪 Testing

Text-only change.

✅ Checklist

• [ ] This PR has an informative and human-readable title.
• [ ] Changes are limited to a single goal - eschew scope creep!
• [ ] All future TODOs are captured in issues, which are referenced in code comments.
• [ ] All relevant type-of-change labels have been added.
• [ ] I have read the CONTRIBUTING document.
• [ ] These code changes follow cisagov code standards.
• [ ] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [ ] Tests have been added and/or modified to cover the changes in this PR.
• [ ] All new and existing tests pass.