Autogenerated Table of Contents

[chillu]

There's some very large docs, such as https://docs.silverstripe.org/en/4/upgrading/. We used a TOC at the top to help devs navigate those docs. While that can be achieved on a per-doc level through manual lists at the top, I think most documents will benefit from having that automated. I think that used to be the case in the old docs site, but not 100% on that.

If we do this, we should ensure that there's no double ups where docs already created their own manual TOC. That's unlikely if it was indeed auto-generated on the old docs site.