doc: improve issue template

doc: improve issue template

The previous template used an ordered list, but the formatting always
breaks when users paste quoted snippets of code or command outputs.
It is also harder to visually parse because items in ordered lists
are only indented but not highlighted in any way.

Change-Id: I73c89e9f0465aef41093f5c54d11bb0d12ff8c8d
Reviewed-on: https://go-review.googlesource.com/27252
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>