merge formats directories into one single markup directory structure

[hboutemy]

Improvement proposal

with #573 , we're trying to get rid of old APT format because it is now unknown to most people: great

IMHO, one key issue we have with mixing formats is that it has separate directory structures, which makes it hard to find source markup corresponding to visible html url: I created https://issues.apache.org/jira/browse/MSITE-1036 to support multiple markup formats in one single directory

I'm not sure we'll be able to really drop every format (xdoc may remain useful in a few places, or even a small remaining part of apt)
with one single directory structure, once we benefit fro MSITE-1036, this will not be such a big issue as currently