Update edx-documentation README

[sarina]

Update the README to correct "EdX" capitalizations, remove outdated links, and explain our move to docs.openedx.org

@feanil I'm happy to do a cleanup pass, but probably can't add the detail you can about the moving strategy/timeline

[feanil]

Happy to do a content pass after you do a cleanup. Most of the rational for the docs.openedx.org move is captured at https://docs.openedx.org/en/latest/documentors/decisions/0001-purpose-of-this-repo.html you can probably link to there for the strategy. As for timeline, we don't currently have one for all of this content to be moved.