weird rendering of the training stuff now

[aineniamh]

[bede]

Since 7e61687

[rambaut]

Training overview now picks up documents with category: guide and category: tutorial. I don't know if we want the mev and mpox ones listed here (if not then we can change their category). "Why sequence measles" could be the sort of guide we want here.

The issues are the standardisation of the top matter fields. Need some generic icons