Docs: responsive embeds and images CSS suggestion

I thought it might be helpful to suggest some CSS in conjunction with the rich text template tag to make embeds/images responsive. The image tag and embed filters provide pixel width control, but the rich text filter does not, so a CSS-inexperienced user might struggle with ways to control output width with that tag.

It also occurs to me that the embed filters are not documented, along with including embeds in models. I plan to add those in a later PR once I've tested and understood them.