Add tags for the content

[LauWien]

The use of tags for the teaching & learning material (e.g. level, duration) could support the navigation on the ece-teaching website. To implement this, the following steps are needed:

1. Finding out if tags can be implemented in a Sphinx HTML build (similar to GitHub issues).
2. Finding out if the content of the page can me sorted by these tags.
3. Implement tags:
   • duration/extent (single lecture to seminar to full credit course);
   • level (beginner to advanced);
   • recurrence;
   • closed/open.