change date do 2021-06-28

This also renames the category of the diagrams post in order to be in
line with the other tutorials.

Update 2021-06-10-story-hts.md, repositioned paragraph about tests from HelmholtzAI section to 'what has been done'

Update 2021-06-10-story-hts.md, added detailed numbers and enhanced the information in the HTS blog post.