Minor tweaks to wording/style

This is a minor pull request that:

Updates the aims section so that it talks directly to the participants (since they are the target audience for the materials).
Removes the duplicate "workshop" from the path for all workshop pages (e.g. so https://wfdb.io/mimic_wfdb_tutorials/workshop/workshop-aims.html becomes https://wfdb.io/mimic_wfdb_tutorials/workshop/aims.html)

wfdb / mimic_wfdb_tutorials