Add Issue and Pull Request Templates

Created by: jmgate

A few months back I whipped up a handful of issue and merge requests templates to use in some of my GitLab projects. Unfortunately GitHub only allows a single template for either issues or pull requests, so I've consolidated the ones I've used before and tried to make them generic enough that they apply to bug reports, feature requests, documentation issues, discussions, or just arbitrary tasks. These templates were pieced together taking inspiration from various best practices examples I found around the web, but I'm afraid I didn't keep track of my sources when putting them together.

On the pull request side of things, it would be a good idea for us to actually fill out CONTRIBUTING.md so contributors know what's expected of them. Perhaps this is part of the ongoing pull request auto-testing work; if not, it should be.

I'm more than happy to iterate on the design here. Since they just pre-populate the Description field when creating an issue or pull request, a user can easily Ctrl+A, Delete, but the goal here is to stack the deck in our favor such that we receive all the information necessary to tackle an issue/pull request at the outset, rather than sending questions back and forth for a week before we have enough information to go on.

@trilinos/framework