Use asciidoc and xmlto to automatic generation of man and html docs

This has the merit to put all the documentation of the project into a
single banner, and to centralize all the project in a single place at
code level.

Compiling documentation can be made by setting the variables ASCIIDOC
and XMLTO. As PostgreSQL extension system is not that smart for doc
generation, some custom Makefile path is used to install man pages into
a folder that could directly by used in MANPATH.