[GitHub Desktop] Stashing changes feature

[ethanpalm]

Why:

Closes #1202

This PR creates a new article, "Stashing changes," to document a new feature and moves content about stashing changes from "Managing branches" to the new article.

What's being changed:

I used two reusables/two steps for the new procedure on stashing changes rather than one so that click-changed-files-header.md can be reused if the Discard All Changes feature is documented later.

Article in staging	Source	Changes
"Stashing changes"	Source	Create new article
index.md	Source	Update index
"Managing branches"	Source	Move content about restoring stashed changes to new article

Check off the following:

• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[lecoursen]

Let me know if you have any questions!

[hubwriter]

This looks good. 👍

I've made a few small inline suggestions. Just a couple of other things:

A few of the images (particularly the Mac versions) result in the page being quite spacey. You could crop <OS>-changed-file-header.png and <OS>--stash-all-changes.png to lose the empty bottom part of the image. Also (although I'm nit-picking here) you could lose some of the white space at the bottom of <OS>-restore-stashed-changes-button.png and <OS>discard-stashed-changes-button.png, just to tighten up the page a little.

It looks like 3 of the reusables you've created are only used once. This works okay but I'd recommend only creating reusables when you're using them more than once. I'd suggest not creating reusables for possible future reuse.

The new page is looking looking really good. Nice work. 🎖️

[shati-patel]

Great, this looks like a useful new article! ✨

One small suggestion, otherwise 👍🏽 (I'm happy to re-approve if the suggestion dismisses this review 😉)

[shati-patel]

Thanks! 🎉