Doc numerous undo possibilities in git

What does this MR do?

Outline of Undo changes in Git tutorial as in gitlab-com/blog-posts#97 (closed) was given some more details, along with general structure and style of the tutorial.

n this tutorial, we will show you different ways of undoing in Git, for which we will assume you have a basic working knowledge of.

We will explain a few different techniques to undo your changes based on the stage of the change in your current development.

More details and previous progress on gitlab-com/www-gitlab-com!4197 (closed)

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
• If working on CE, submit an MR to EE with the changes as well.

Closes gitlab-com/www-gitlab-com!4197 (closed) gitlab-com/blog-posts#97 (closed) https://gitlab.com/gitlab-com/community-writers/issues/40

@marcia