[MERGE] change extension of generated html documentation from htm to html

[Ian Clatworthy]

John Arbash Meinel wrote:

> The NEWS file is intended to be in reST syntax, just like the rest of our docs.
> It isn't 100%, but I've gone through and cleaned it up from time to time.
> 
> It seems reasonable to generate a NEWS.html with the rest. Though I'm not sure
> where it would be put.

My proposal buried in my RFC yesterday was for NEWS.html to go in
doc/en/release-notes/.

Ian C.