Emphasise importance of not embedding text in images/video

The [AMP Story Best Practices Guide](https://amp.dev/documentation/guides-and-tutorials/develop/amp_story_best_practices) should emphasise that for multiple reasons--discoverability, accessibility, performance, responsiveness, etc.--text should not be embedded into images or videos. (i.e. layers should not be "flattened", text should not be burnt into videos.)

Adding a section to the the "Technical" section is probably fine, but ideally it shouldn't be possible to read e.g. the [Animation](https://amp.dev/documentation/guides-and-tutorials/develop/amp_story_best_practices#animation) section in isolation and think that's it's totally fine to produce those effects via video.

/cc @newmuis