doc: use https:// in github issue URLs

Signed-off-by: David Aguilar <davvid@gmail.com>
doc: remote "closed/#issue/" from issue URLs

Signed-off-by: David Aguilar <davvid@gmail.com>
doc: mention "push to multiple remotes" in the v1.8.6 release notes

Signed-off-by: David Aguilar <davvid@gmail.com>