.github: add default PR template

Based on the .github/PULL_REQUEST_TEMPLATE file in the main Go repo
at commit e4283592e5cb00d84544df2442b0d3f4eccf8c72 (go.dev/cl/777780),
with modifications to make it a better fit for most golang.org/x repos.

- changed the wiki page link

Fixes golang/go#79215

Change-Id: I68dbaa75f384fce54a8ab502b82cdfb341b53766
Reviewed-on: https://go-review.googlesource.com/c/dot-github/+/781541
TryBot-Bypass: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>