docs: explicitly ask for PR template checklist being true and checked

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing issue: fixes Add PR template that asks for an attached issue #4010
• That issue was marked as accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Makes the PR template very clear that you must have an associated issue.

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✅ Successfully ran 48 targets

• Node 12.x - nx run-many --target=typecheck --all --parallel
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• Node 14.x - nx test @typescript-eslint/utils
• Node 12.x - nx run-many --target=build --all --parallel --exclude website --exclude website
• Node 12.x - nx test @typescript-eslint/parser
• Node 14.x - nx test @typescript-eslint/scope-manager
• Node 12.x - nx test @typescript-eslint/type-utils
• Node 12.x - nx test @typescript-eslint/utils
• Node 14.x - nx test @typescript-eslint/visitor-keys
• Node 12.x - nx test @typescript-eslint/scope-manager
• Node 14.x - nx test @typescript-eslint/typescript-estree
• Node 16.x - nx prebuild @typescript-eslint/types
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• Node 14.x - nx run-many --target=build --all --parallel --exclude website --exclude website
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• Node 12.x - nx prebuild @typescript-eslint/types
• Node 12.x - nx prebuild @typescript-eslint/types
• Node 14.x - nx prebuild @typescript-eslint/types

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[typescript-eslint]

Thanks for the PR, @JoshuaKGoldberg!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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