Update concept section of "changes" page

What does this MR do?

This page (https://docs.gitlab.com/ee/user/project/merge_requests/changes.html) is becoming more difficult to work on without a clearer breakdown of all the UI elements contained in a diff. @msedlakjakubowski is right that we need documentation for the pinned-file feature (#387246 (closed)) but I need to fix the page's structure to support those docs, first.

Still very VERY draft status at this time. My terminology is rough / wrong, and right now it's a big pile of lines.

Related issues

• Related to [Feedback] Pinned file (#439582)
• Related to Pin file when linking to a file line in an MR (#387246 - closed)

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