Add a page to describe the vdm language(s)

[lausdahl]

We need a page that describes the VDM language by:

• first describing the basic VDM of the iso standard, types, functions etc.
• a short section that describes modules
• a short section that describes the addition of ++
• a short section that describes the addition of RT

[ldcouto]

We have some of this in http://overturetool.org/languages/

But it's incomplete (no RT stuff, for example). I honestly think the best solution here is to create an online version of the language manual. Maybe see what pandoc tex -> markdown produces.

[augustoribeiro]

looking good 👍

[joey-coleman]

@pglvdm Do we have any existing text that would cover this?

[ldcouto]

There's VDM RT content commented out in the source for http://overturetool.org/languages/.

Just needs someone to go in and reorganize the pages.