Describe the bug

In the PR pointers section of the PR template, we mention that reviewers can be assigned by using "{{Question/comment}} @{{reviewer_username}} PTAL". However, contributors often don't notice this. They either:

(a) don't think anything else needs to be done after opening the PR. This results in their PR sitting around without reviewers for a prolonged period, after which it gets auto-closed by Oppiabot.
(b) realize that they need to assign a reviewer, but have no idea how to do so. It is up to chance whether an existing contributor looks at the PR and either assigns it to a reviewer directly, or points them to that part of the instructions.

In general I'm not sure I've actually ever seen a new contributor who didn't miss this. The majority of successful reviewer assignments are done by Oppiabot or by existing contributors. This results in a hole in our "first PR" experience.

Steps To Reproduce

1. Create a PR.

Expected Behavior

The PR should be assigned automatically to the correct reviewers (note that the new contributor typically doesn't know who the correct reviewers would be, and the filer of the issue is not necessarily going to be the reviewer).

If there is no reviewer assigned then someone on the dev workflow team should be informed so that they can fix the situation.

The information about how to ask Oppiabot to assign someone should also probably be made more prominent, both in the PR instructions (or PR thread, perhaps as a separate post from Oppiabot), as well as the wiki pages encountered by a newer contributor when getting started.

Screenshots/Videos

No response

What device are you using?

No response

Operating System

No response

What browsers are you seeing the problem on?

No response

Browser version

No response

Additional context

No response