Add Structured Data on Commons support to index page

Mention it in the introductory paragraph (now two paragraphs), and add
another form where people can enter a file title to be redirected to the
correct /file/… URL. This form is placed above the one for IIIF region,
since I think it’s more important than that one (the IIIF region support
is mainly for having this tool as the formatter URL of the “relative
position within image” property, not so much for interactive use).