Document plugins / themes installed via Composer (#1037)

Page `feature-focus/composer` should contain some info about this feature. Also, there is a link from [code](https://github.com/versionpress/versionpress/blob/84d45b8918895a36a1a3c90683f6a82cfb7f91f7/plugins/versionpress/src/Utils/RequirementsChecker.php#L129) to this page.
