Update index.md

[RaquelPinho]

Added slides for Erich Schwarz, lecture.

PR author:

• pull request is ready for review & merge

Things to check:

• tutorial resets working directory at beginning with a `cd ~/``
• tutorial contains relevant conda install -y commands at the beginning
• tutorial links to previous tutorial (at top) and next tutorial (at bottom)
• tutorial has learning objectives at top

[karenword]

Approving for immediate use -- will follow up by adding pdf files to github repo (since I'm not sure they are permanent on Slack)

[charlesreid1]

Erich's slides were posted to OSF (since the PDF file was >25 MB) and the link to those slides have been added to the index.

Also see #403 (which added the files, but was closed) and #404 (which added the OSF link and has been merged).