Rollup merge of #123492 - lcnr:master, r=davidtwco

add pull request template asking for relevant tracking issues

As mentioned at RustNation, I would like to remind PR authors to link to relevant tracking issues when opening PRs as it is otherwise very easy to forget doing so.

There's a certain amount of conflict between making the template as small as possible while still being clear for new contributors. I am very much open to changes here but really want to try this out.

Also unsure how much formal buy-in we need here. Maybe merge this pinging t-compiler and t-libs, and then ask how people feel about this on zulip in a few weeks?

r? `@davidtwco`

.github/pull_request_template.md

@@ -0,0 +1,10 @@
+<!--
+If this PR is related to an unstable feature or an otherwise tracked effort,
+please link to the relevant tracking issue here. If you don't know of a related
+tracking issue or there are none, feel free to ignore this.
+
+This PR will get automatically assigned to a reviewer. In case you would like
+a specific user to review your work, you can assign it to them by using
+
+    r​? <reviewer name>
+-->