[ty] contribution guide (#18061)

First take on a contributing guide for `ty`. Lots of it is copied from
the existing Ruff contribution guide.

I've put this in Ruff repo, since I think a contributing guide belongs
where the code is. I also updated the Ruff contributing guide to link to
the `ty` one.

Once this is merged, we can also add a link from the `CONTRIBUTING.md`
in ty repo (which focuses on making contributions to things that are
actually in the ty repo), to this guide.

I also updated the pull request template to mention that it might be a
ty PR, and mention the `[ty]` PR title prefix.

Feel free to update/modify/merge this PR before I'm awake tomorrow.

---------

Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>
Co-authored-by: David Peter <mail@david-peter.de>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,9 @@
 <!--
-Thank you for contributing to Ruff! To help us out with reviewing, please consider the following:
+Thank you for contributing to Ruff/ty! To help us out with reviewing, please consider the following:
 
 - Does this pull request include a summary of the change? (See below.)
-- Does this pull request include a descriptive title?
+- Does this pull request include a descriptive title? (Please prefix with `[ty]` for ty pull
+  requests.)
 - Does this pull request include references to any relevant issues?
 -->