Module:Sister project links/doc

Implements Sister project links

See Sister project links/testcases for test cases for box, Sister bar/testcases for bar.

Note: in order to make the test cases work, the Sandbox CSS classes have "-sand" appended to their names. If you wish to update the CSS, copy the contents of each class from Module:Sister project links/sandbox/styles.css to Module:Sister project links/styles.css, but do not alter the class names, nor just copy-paste the entire CSS file. For the current difference in CSS between Sandbox and Main, see here.