Update PR template

Author: brandonwillard

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,14 +1,21 @@
+<!-- **Thank you for opening a PR!** -->
 
-**Thank you for opening a PR!**
+## Description
 
-Here are a few important guidelines and requirements to check before your PR can be merged:
+<!-- Include a description of the changes and provide references to the relevant issues and/or discussions. -->
+
+## Checklist
+
+<!-- Here are a few important guidelines and requirements to check before your PR can be merged: -->
+
++ [ ] You have read [CONTRIBUTING.md](https://github.com/aesara-devs/aesara/blob/main/.github/CONTRIBUTING.md), the [developer start guide](https://aesara.readthedocs.io/en/latest/dev_start_guide.html), and any documentation relevant to the proposed changes (e.g. [Creating a new `Op`](https://aesara.readthedocs.io/en/latest/extend/op/creating_an_op.html) when proposing a new `Op`).
 + [ ] There is an informative high-level description of the changes.
 + [ ] The description and/or commit message(s) references the relevant GitHub issue(s).
 + [ ] [`pre-commit`](https://pre-commit.com/#installation) is installed and [set up](https://pre-commit.com/#3-install-the-git-hook-scripts).
 + [ ] The commit messages follow [these guidelines](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
 + [ ] The commits correspond to [_relevant logical changes_](https://wiki.openstack.org/wiki/GitCommitMessages#Structural_split_of_changes), and there are **no commits that fix changes introduced by other commits in the same branch/BR**.
 + [ ] There are tests covering the changes introduced in the PR.
 
-Don't worry, your PR doesn't need to be in perfect order to submit it.  As development progresses and/or reviewers request changes, you can always [rewrite the history](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History#_rewriting_history) of your feature/PR branches.
+<!-- Don't worry, your PR doesn't need to be in perfect order to submit it.  As development progresses and/or reviewers request changes, you can always [rewrite the history](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History#_rewriting_history) of your feature/PR branches.
 
-If your PR is an ongoing effort and you would like to involve us in the process, simply make it a [draft PR](https://docs.github.com/en/free-pro-team@latest/github/collaborating-with-issues-and-pull-requests/about-pull-requests#draft-pull-requests).
+If your PR is an ongoing effort and you would like to involve us in the process, simply make it a [draft PR](https://docs.github.com/en/free-pro-team@latest/github/collaborating-with-issues-and-pull-requests/about-pull-requests#draft-pull-requests). -->