Define the minimum requirements for a media element

[dylanjha]

1. The entire Video element API not not required to for a media elements, let's define explicitly what subset is required (maybe a media-element.md file in docs/) ?
2. Let's also make sure we have some test coverage to make sure a minimal video element works on the normal happy paths

Discussion happened in this thread: #93 (comment)

[cjpillsbury]

Should we start this in https://github.com/muxinc/media-chrome/discussions/categories/proposals ?