Document the new replace-at-render option

[annegentle]

You can now replace text based on the NUC in use when the lab is rendered.

Example repo here: https://github.com/patrickcisco/sample-replace-at-render

Example output on staging, showing the use of Event mode:
https://learninglabs.cisco.com:8867/lab/sample-replace-at-render/step/1