Add `git` instructions to bug report and feature issue forms

[rebecca-shoptaw]

Closes #9269.

Feature. Adds instructions re: basic git pull/push operations to the issue template -- should help contributors avoid merge conflicts/bizarre issues/confusion as a result of out-of-date branches.

Technical

• I left out the "suggested branch name" section that was originally suggested for now, to avoid giving submitters more to do, and to make the new section as simple and legible as possible -- but happy to add it back in if people have been having issues with badly-named branches!
• Added --ff-only to the pull instructions (thanks @tfmorris!) in the Git Cheat Sheet
• I couldn't immediately find a way to make a non-editable section of the template (and as we discussed, there are some cases where the instructions wouldn't be necessary), so I left it editable but with a note re: leaving it as is, which hopefully only experienced contributors will ignore.
• Happy to take any more input re: wording, placement, etc.!

Testing

1. Make a new GitHub issue for a feature or bug
2. You should see a new reminder re: keeping branches up to date at the bottom of the issue, both in the form and in the submitted version

Screenshot

Current Implementation:

Previous implementation:

Screenshot via a markdown previewer.

Stakeholders

@RayBB @merwhite11 @mekarpeles