rustdoc: wrap "source" link to new line on narrow screens

[jsha]

Following up #92861, where we let the out-of-band info (1.0.0 · source · [-]) move to a new line on narrow screens, we should give the same treatment to the individual source links next to methods and impl blocks. Note this will require a bit of a change in the DOM, to put the source links after the headings. Like the previous PR, we should set flex-wrap, so they can wrap on an individual basis as needed, plus a flex-direction: column in the mobile settings, so past a certain narrowness they always get wrapped.