[v3] allocator documentation revamp

[Benjamin Kosnik]

>Do you want to keep that change? If so it will have to be put back
>manually every time documentation.html is regenerated. The simplest
>solution is to use the generated version, but this means an extra click
>to get to the allocator docs from the front page.

I'd prefer the change, but at the same time I'm not up for manually
re-inserting it after every regeneration. So, this is fine, I think.

-benjamin