Add nice thumbnails to resources

[deepthi]

In #1886 we re-worked the resources page. It looks much better now :)
I noticed that while all the YouTube videos have nice thumbnails, some of the other content (podcasts/slides/non-YouTube hosted videos), don't. It will be nice to add those.
You can see that here.

[deepthi]

The yaml file actually tells you how to generate a thumbnail_url, but it doesn't work for everything. For instance, I tried it with the "Life of a Query..." video and it said "Maximum file size exceeded"

I did generate one for the first podcast on that list though, and here it is

[thisisobate]

I tried it with the "Life of a Query..." video and it said "Maximum file size exceeded"

For cases like this, I guess it's safe to use the vitess logo as a fallback right?

[thisisobate]

Btw, feel free to assign me to this one