Improve screen reader experience, version history

Our collapsed version history button (https://docs.gitlab.com/ee/user/clusters/agent/ has an example) has a button to expand the history, but the button contains no text. In the gitlab-docs repo it's built by lib/filters/introduced_in.rb. I tried adding a title attribute to the button, but after talking to @jElder the better answer is something like aria-label="Version history".

This merge request adds an aria-label to the element, making it accessible and comprehensible to users of screen readers.