Update site to accommodate 3.x documentation

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.

Start by reading `_config.yml`, the `_docs-2/` collection, `_docs-2/configuration/`, `_docs-2/apidocs`, and the `_plugins/links.rb` plugin to understand how the current 2.x docs and convenience links are assembled. The issue leaves the approach open: either support version-specific shared docs or add a separate 3.x collection. Done means the website supports both 2.x and 3.x docs, including the links.

Written by the indexing model from the issue text.