dmd: Some improvements to the dmd manual

[Alex Sassmannshausen]

Hello,

I'm assuming that dmd contributions go to the guix list for now.

Attached a patch introducing some changes to the dmd manual — the
patch only covers the Introduction section for now as I wanted to
submit something with my approach early to know whether it is
acceptable.

As you can see it's a bit of a rewrite, but I think the English is
overall easier to understand.

Let me know if you think I should ba approaching this differently.

Best wishes,

Alex