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  

See merge request !4324