Streamline PR template and add CONTRIBUTING.md link (#3624)

Empirically, many of the sections of the current PR template are being
left empty. I wanted to propose a more minimal template that also points
to CONTRIBUTING.md (which needs to be worked on to add a new contributor
guide). It also puts the "instructions" in Markdown comments (so they
are hidden in the UI if the contributor does not delete them) to make
rendered PR descriptions easier to read. I also question whether the QA
notes section is needed, since QA is usually looking at issues when
doing verification rather than pull requests. So QA notes may be better
off there.

---------

Signed-off-by: Wes McKinney <[email protected]>
Co-authored-by: Julia Silge <[email protected]>

.github/pull_request_template.md

@@ -1,15 +1,26 @@
 <!-- Thank you for submitting a pull request.
-Please ensure that the code is up-to-date with the `main` branch.
--->
-
-### Intent
-
-> Describe briefly what problem this pull request resolves, or what new feature it introduces. Include screenshots of any new or altered UI. Link to any GitHub issues that are related.
+If this is your first pull request you can find information about
+contributing here:
+  * https://github.com/posit-dev/positron/blob/main/CONTRIBUTING.md
 
-### Approach
+We recommend synchronizing your branch with the latest changes in the
+main branch by either pulling or rebasing.
+-->
 
-> Describe the approach taken and the tradeoffs involved if non-obvious; add an overview of the solution if it's complicated.
+<!--
+  Describe briefly what problem this pull request resolves, or what
+  new feature it introduces. Include screenshots of any new or altered
+  UI. Link to any GitHub issues but avoid "magic" keywords that will 
+  automatically close the issue. If there are any details about your 
+  approach that are unintuitive or you want to draw attention to, please 
+  describe them here.
+-->
 
 ### QA Notes
 
-> Add additional information for QA on how to validate the change, paying special attention to the level of risk, adjacent areas that could be affected by the change, and any important contextual information not present in the linked issues.
+<!--
+  Add additional information for QA on how to validate the change,
+  paying special attention to the level of risk, adjacent areas that
+  could be affected by the change, and any important contextual
+  information not present in the linked issues.
+-->