chore: remove HTML comments and inline guide

Also make template lighter.

See
https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/pr.20template/with/521309075

Author: weihanglo

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,36 +1,12 @@
-<!--
-Thanks for submitting a pull request 🎉! Here are some tips for you:
-
-* If this is your first contribution, read "Cargo Contribution Guide" first:
-  https://doc.crates.io/contrib/
-* Run `cargo fmt --all` to format your code changes.
-* Small commits and pull requests are always preferable and easy to review.
-* If your idea is large and needs feedback from the community, read how:
-  https://doc.crates.io/contrib/process/#working-on-large-features
-* Cargo takes care of compatibility. Read our design principles:
-  https://doc.crates.io/contrib/design.html
-* When changing help text of cargo commands, follow the steps to generate docs:
-  https://github.com/rust-lang/cargo/tree/master/src/doc#building-the-man-pages
-* If your PR is not finished, set it as "draft" PR or add "WIP" in its title.
-* It's ok to use the CI resources to test your PR, but please don't abuse them.
+Thanks for the pull request 🎉!
+Please read the contribution guide: <https://doc.crates.io/contrib/>.
 
 ### What does this PR try to resolve?
 
 Explain the motivation behind this change.
 A clear overview along with an in-depth explanation are helpful.
 
-You can use `Fixes #<issue number>` to associate this PR to an existing issue.
-
-### How should we test and review this PR?
+### How to test and review this PR?
 
 Demonstrate how you test this change and guide reviewers through your PR.
 With a smooth review process, a pull request usually gets reviewed quicker.
-
-If you don't know how to write and run your tests, please read the guide:
-https://doc.crates.io/contrib/tests
-
-### Additional information
-
-Other information you want to mention in this PR, such as prior arts,
-future extensions, an unresolved problem, or a TODO list.
--->