Add structured data for Search Engines

[lorenzschmid]

Similarly to #13 improving sharing of links to the template, this issue tackles a better integration for search engines: Adding so called Structured Data to the HTML files allows search engines such as Google to recognize that this is a website for an event and include useful direct links, e.g. to the ticket platform. Unfortunately, this feature is currently not available in many countries and thus the issue is not pressing.