BUG/SMALL - Note use of github links on the expand-assets page

[becky-gilbert]

Make a note on the expand-assets docs page to remind people to use the raw.githubusercontent.com link when using the expand-assets method.
See slack discussion: https://lookit-mit.slack.com/archives/CHG0M1LA1/p1659984109952199
And lookit-stimuli-template readme: https://github.com/lookit/lookit-stimuli-template#accessing-your-stimuli-and-telling-lookit-where-to-find-them-or-what-do-i-put-for-basedir

[becky-gilbert]

There's still a lot of confusion over how to link to stimuli files. We recently had two support requests where users were (a) using expand assets along with a full path as the video source, and (b) using expand assets but not testing the files under the file type subdirectories. We probably need to clarify the docs on this!

In particular, we should put all of the relevant info in a single page and make it very easy to find. And we should include clear/thorough examples of how to set up each stimuli hosting/linking method.