Consider different page layouts for rustdoc

[jyn514]

@willcrichton and I were discussing this on discord:

wcrichto: auto-generated docs tend to have a single, immutable structure. But most effective data visualization tools present multiple views on the same data, depending on the task at hand. Would it make sense for rustdoc to have multiple layouts? And the user could pick the one that makes the most sense for their question.
jynelson: sure :) we could even make the default layout configurable, we have settings already
wcrichto: One possibility would be to improve the visual design of the docs to more easily give an overview of all the methods on a type. In the split_at_mut, even literally just putting all the methods into 2D grid instead of a 1D list could be helpful. There are ~180 methods on the Vec documentation, which is overwhelming to look through.
jynelson: you mean something like 'Method Summary' and 'Field Summary' from javadocs?
wcrichto: Yeah maybe! But with basic grouping/filtering. Like: "group by whether method takes &self or &mut self".

Grouping by &self/&mut self/self would be a neat grouping, but I'm sure there are many others possible. It would be great to have different ways of visualizing how to manipulate types.

Related: type-based searching (seems to be currently broken? #60485 (comment))