Update interface-guidelines cronjob to match the new in-tree path and styling

[waddlesplash]

In hrev48796, I migrated the interface-guidelines document to docs/ from src/documentation, as well as making it static instead of dynamic. The cronjob that updates the copy at ​http://api.haiku-os.org/HIG/ needs to be updated to account for this.

(Eventually it will be split into multiple pages, so copying the entire directory is the thing to do.)