rustdoc renders items synchronously

[rylev]

Currently rustdoc is rendering every item synchronously meaning that when there are many, many items, it takes longer than it would if these items could be processed in parallel. Moving this to being parallel won't be easy as clean::Item and html::rendered::Context are not Send for many different reasons some of which include the fact that they use datastructures from rustc that are not Send (which will rely on turning on parallel compilation to get working).

First steps to improving this is to find out where most of the time in rendering is being spent and improving that.

[jyn514]

This and #82741 are basically the same issue, but #82741 has more detail, so closing this as a duplicate.