Make it easier to get to the related_to hierarchy in the docs

[cbizon]

From the front page of the docs, it's difficult to get to the related to hierarchy.

For a lot of teams, getting the classes and the predicates are the parts that they want the most often, so those should be the easiest to get to.

The simplest way I found to get to related to was to search for "related_to". searching for "related to" (no underscore) gave a bunch of less relevant results.