chore: add disclaimer to PR template

[dreamorosi]

Description of your changes

As discussed in the linked issue, there was room for improvement in the communication around PRs and issues at the time of contributing. This PR adds a disclaimer to the PR template that aims at making more explicit to contributors that a PR should be preceded by a triaged issue.

This PR resolves #1350.

How to verify this change

See the bottom of this PR description, where I have manually pasted the same markdown that I have added to the template. The PR template will be in effect starting from the next PR after this one is merged.

Related issues, RFCs

Issue number: #1350

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published
• The PR title follows the conventional commit semantics

Breaking change checklist

Is it a breaking change?: NO

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Disclaimer: We value your time and bandwidth. As such, any pull requests created on non-triaged issues might not be successful.