Adding Toc for the "longish" spec pages.

[Seb]

On Friday, 29 December 2017 at 12:17:47 UTC, Seb wrote:
> On Friday, 29 December 2017 at 12:13:02 UTC, Mike Franklin 
> wrote:
>> [...]
>
> Yes, opening an issue or PR is the best way to move things 
> forward.
> In this case, this is already on my radar and will happen soon 
> (since a couple of weeks we already got a footer pagination). 
> It's pretty ugly because Ddoc is used for dlang.org which makes 
> trivial stuff like this rather complicated :/

TOC generation is there -> 
https://github.com/dlang/dlang.org/pull/2043