Update Docker docs for 6.0.0 and image flavours
* Use Compose v2 in Docker examples
* Use US spelling in Docker docs
* Link to www.docker.elastic.co in docs
This overhaul of the docs structure puts Kibana's documentation more
inline with the structure that is used in Elasticsearch. This will help
us better organize the docs going forward as more docs are added.
This also includes a few necessary content changes for 5.0.