PUBDEV-4360: Add links for SW 2.1 docs on documentation site (#1082)
PUBDEV-4360: Add links for SW 2.1 docs on documentation site (#1082)

On the docs.h2o.ai site, replaced SW 1.6 documentation links with SW
2.1. This now matches the SW download page, which only includes 2.0 and
2.1.
h2oai/h2o-dev,spennihana/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,h2oai/h2o-dev,mathemage/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,mathemage/h2o-3,h2oai/h2o-dev,spennihana/h2o-3,h2oai/h2o-3,spennihana/h2o-3,h2oai/h2o-3,spennihana/h2o-3,mathemage/h2o-3,mathemage/h2o-3,h2oai/h2o-3,h2oai/h2o-3,spennihana/h2o-3,michalkurka/h2o-3,mathemage/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,spennihana/h2o-3,mathemage/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,spennihana/h2o-3,mathemage/h2o-3