Simplify pull-request template (#15924)

The current template is quite wordy and hard to read.  We have a
properly rendered checklist in `CONTRIBUTING.md`, so it's clearest just
to link to that; I'm not convinced that the existing comment actually
drives user behaviour to do what we want, so I don't think we'll lose
out by putting it a link away.

I deleted the "summary" and "details and comments" section headings
because they're usually below the fold of the text area anyway, and just
busy-work to delete for PRs that come with a good commit message.

One thing we definitely _do_ want in all PR descriptions, and is not
possible for maintainers to manually do themselves, is to include a
disclosure of the AI/LLM tooling used.  This new template pre-populates
that with tickboxes, including for the negative case, which should at
least make it obvious to reviewers if all the information is present.

Author: jakelishman

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,20 +1,13 @@
 <!--
-⚠️  If you do not respect this template, your pull request will be closed.
-⚠️  Your pull request title should be short detailed and understandable for all.
-⚠️  Also, please add a release note file using reno if the change needs to be documented in the release notes.
-⚠️  If your pull request fixes an open issue, please link to the issue. Use "Fixes #XXXX" if this PR *fully* closes the issue XXXX.  
-☢️  If you used an AI tool to code this PR, add "AI tool used: <Name and version of the tool>". For example, "AI tool used: Microsoft Copilot Chat with GPT-5". Failing to disclose the use of AI tools may result in the PR being closed without further review.  
-
-
-- [ ] I have added the tests to cover my changes.
-- [ ] I have updated the documentation accordingly.
-- [ ] I have read the CONTRIBUTING document.
+ * See https://github.com/Qiskit/qiskit/blob/main/CONTRIBUTING.md#pull-request-checklist
+ * Write a clear description here.
+ * Use "Fix #15919" to close issues.
 -->
 
-### Summary
-
-
-
-### Details and comments
+### AI/LLM disclosure
 
+- [ ] I didn't use LLM tooling, or only used it privately.
+- [ ] I used the following tool to help write this PR description:
+- [ ] I used the following tool to generate or modify code:
 
+<!-- Any code generated by LLM or modified from LLM suggestions must commented inline too. -->