Replace custom script with existing MkDocs plugin

[galund]

Simplification, along with fix for #54

The plugin has some issues, but this does provide a fairly simple fix for the issues in the menu.

There's still a duplication of the site name at the top of the menu (which looks odd at some screen resolutions only!) but this is maybe good enough.

[GDS-Dominic]

I am wary about this whilst less code could be quicker and easier to maintain I think current script provides us some useful flexibility i.e. in my current branch for audit events i've been able to do a lot more manipulation with it which has come in useful

[galund]

I think we can build a structure like that pretty easily... which branch, I don't mind having a play?

If we do decide we want something custom then I think it might be worth writing it into an extension rather than overwriting the nav file - and I started looking at that before realising that someone had in fact already built something that does the job.