Update the pages that will appear in the header

[github-actions]

Update the pages that will appear in the header (header_pages:) in _config.yaml. Any page that appears in the header should have a navigation title (nav_title:) in its YAML header.

[jashapiro]

Just a note here that we may want to have a quick think about what is in this list: we often (maybe more in Single Cell) get requests for how to work with your own data, which has its own page, so we should make sure that is prominent. Looking at the page, this might rename "Consultation Sessions" as "Resources", but also link to https://alexslemonade.github.io/2021-march-training/working-with-your-data/working-with-your-own-data.html