integration with is-mirage-broken

added files that will be modified, or relied upon to exist, by the update_mirage-www.sh script at https://github.com/mirage/is-mirage-broken/pull/12

Summary of changes:
- update_mirage-www.sh is going to make PRs to update a wiki page in mirage-www:  tmpl/wiki/is_mirage_broken.md
- tmpl/wiki/index.md has been modified to include the new wiki page
- src/data.ml has been modified to include the metadata of this new wiki page. (src/data.ml will also be updated by update_mirage-www.sh to freshen the date of the wiki page.)