Update PR template and dev process docs (#6158)

Overall, I'm just trying to make these bits of documentation reflect our
process as it stands today. There are some specific changes I want to
draw attention to though.

Asking new contributors to pick a reviewer is a waste of time for two
reasons: Only people with write access to the repository are allowed to
pick reviewers, and new contributors have no idea who would be a good
reviewer for their PR anyway. So I'm deleting all mention of that. We
now auto-assign reviewers instead.

By the time someone is opening a PR, asking them to open an issue just
makes extra work for everyone. They've already picked an approach
without discussing it; we might as well look at what they did. We may
then have to ask them to take a different approach, but at that point,
asking them to open an issue won't save them any effort.

I removed mention of tests from the pull request template. There are
many things we'd like to see in a PR, and we may have to ask for them
during review if the contributor doesn't follow our development process
documentation. But I think the only crucial information for starting a
review is the two questions I'm leaving in the template: why do you want
this, and where can I find more context?

The code of conduct link still had the branch name as `master`, which is
a hint at how long it's been since anyone reviewed it.

.github/pull_request_template.md

@@ -1,18 +1,15 @@
 <!--
+Please make sure you include the following information:
 
-Please ensure that the following steps are all taken care of before submitting
-the PR.
+- If this work has been discussed elsewhere, please include a link to that
+  conversation. If it was discussed in an issue, just mention "issue #...".
 
-- [ ] This has been discussed in issue #..., or if not, please tell us why
-  here.
-- [ ] A short description of what this does, why it is needed; if the
-  description becomes long, the matter should probably be discussed in an issue
-  first.
-- [ ] This PR contains test cases, if meaningful.
-- [ ] A reviewer from the core maintainer team has been assigned for this PR.
-  If you don't know who could review this, please indicate so. The list of
-  suggested reviewers on the right can help you.
+- Explain why this change is needed. If the details are in an issue already,
+  this can be brief.
 
-Please ensure all communication adheres to the [code of
-conduct](https://github.com/bytecodealliance/wasmtime/blob/master/CODE_OF_CONDUCT.md).
+Our development process is documented in the Wasmtime book:
+https://docs.wasmtime.dev/contributing-development-process.html
+
+Please ensure all communication follows the code of conduct:
+https://github.com/bytecodealliance/wasmtime/blob/main/CODE_OF_CONDUCT.md
 -->