Add a Replay troubleshooting entry for Canvas

[jas-kas]

On this page: https://docs.sentry.io/platforms/javascript/session-replay/troubleshooting/

To deal with this case:

Canvas didn't work out of the box due to an obscure error about our "canvases being tainted", which we were able to fix by setting crossorigin="anonymous" on the images. Here's the documentation that pointed us towards the solution, might be something your SDK can better help folks with (or even just the Sentry docs).