[spec] Second attempt to revise the Introduction

[Dibyendu Majumdar]

Hi,

My previous pull request generated some negative feedback because 
I had adopted some terminology used by the C Standard. In 
particular I was trying to use the term 'translation' rather than 
'compilation' as former is arguably more generic English term. 
However given that D doesn't have a pre-processing stage perhaps 
we can keep using 'compilation' - although in English, outside of 
compilers, compilation means something collecting things so 
translation is a more appropriate word for what the compiler does.

Anyway please review my fresh attempt:

https://github.com/dlang/dlang.org/pull/2650

I hope to add more sections covering Memory Model, Object Model 
and Execution Model but these require some effort so will be in 
later requests.

Thanks and Regards
Dibyendu