streamline contribution guidance

Hello!

From [my initial stumbling](https://github.com/exercism/problem-specifications/pulls?q=is%3Apr+author%3Akatrinleinweber+sort%3Acomments-desc) (1k thanks to @stkent for the hints 🙂 ) and [this](https://github.com/blog/2111-issue-and-pull-request-templates), I gathered that PR templates might be useful for this repo as well.

I **suggest creating a `.github`** folder (and would be willing to do it) for
1. `CONTRIBUTING.md` (just moving it), and
2. a `PULL_REQUEST_TEMPLATE.md`(workflow details extracted from `README.md` and `CONTRIBUTING.md`, for example in the form of a task list).

IMHO, this could bring guidance info to the... Actionable point and time of contact, so to speak. Particularly new contributors may feel that the two existing files are rather a lot to read (me: guilty as charged 😅 ). Seasoned contributors could of course skip over or remove the items, but would also be reminded about the current UX for newbies.

**What do you think?**

PS: Albeit also touching a general topic, this topic seems more ["actionable" here than "_other_"](https://github.com/exercism/discussions#discussions) ;-) PLMK if I should move this issue there.