Add index.md for docs section on workflow GHAs

[sjspielman]

This issue tracks adding the index.md file for a section on OpenScPCA workflows in the new reproducibility docs section. We should provide an overview:

• We have workflows that automate module testing/running, and building dockerfiles to enable those.
• Emphasize that we mostly take care of this, so these docs provide an overview of what's going on under the hood. You can do it too if it's up your alley though!

[sjspielman]

Closed by #533