Add Issue/PR Templates to deter issues/contributions on the GitHub mirror of the project

What does this MR do?

Adds GitHub-specific ISSUE_TEMPLATE.md and PULL_REQUEST_TEMPLATE.md files in a .github directory. To prevent new issues/PRs, I figured it'd be good to direct users/contributors to open issues/contribute code in the "correct" project.

Are there points in the code the reviewer needs to double check?

Wording/phrasing, mostly.

Why was this MR needed?

The GitHub issue tracker is being closed, and PRs on GitHub haven't been accepted for a while now. This was discussed briefly during the GitLab Strategy Session at the Austin Summit.

cc: @dzaporozhets @rymai @MrChrisW @dblessing @virtuacreative @amara