Documentation: Fixing outdated links
Documentation: Fixing outdated links

Removed or replaced outdated links in the Welcome section of the user
guide.

h2oai/h2o-dev,michalkurka/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,h2oai/h2o-3,mathemage/h2o-3,h2oai/h2o-3,h2oai/h2o-3,spennihana/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,mathemage/h2o-3,h2oai/h2o-dev,spennihana/h2o-3,h2oai/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,spennihana/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-dev,spennihana/h2o-3,mathemage/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,h2oai/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,mathemage/h2o-3,mathemage/h2o-3,h2oai/h2o-3,spennihana/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,mathemage/h2o-3,mathemage/h2o-3