Update site to accommodate 3.x documentation

[ctubbsii]

The current docs on the website are specifically 2.x docs, and 3.0.0 removes and changes things, so that we can't just update the docs overwriting what is there, like we could with backwards-compatible 2.x updates. We can either redesign the common docs to allow version-specific documentation, or we can copy over the whole Jekyll docs-2.x collection for 3.x and put them in a different directory. If we do that, we'll have to update the Jekyll ruby plugin to support the convenience links that we had before that currently only work with 2.x.

This involves understanding how the _config.yml file properties, the _docs-2/ collection, the generated client and server properties in _docs-2/configuration/ the _docs-2/apidocs, and the _plugins/links.rb all work together, and updating it all to support both the 2.x docs and the 3.x docs.

[ctubbsii]

See apache/accumulo#3714 (comment)