merge formats directories into one single markup directory structure

### 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

Start by reading issue #573 and the linked MSITE-1036 discussion, then inspect how the repository currently separates APT, xdoc, and other markup sources. Done means the formats share one directory structure without losing necessary xdoc or remaining APT support, but the issue does not name specific files or tests.

Written by the indexing model from the issue text.