Makefile: Use asciidoc and Pandoc to generate MediaWiki doc versions

Makefile: Use asciidoc and Pandoc to generate MediaWiki doc versions

We need this because hosting content under:

  https://www.gentoo.org/proj/

is deprecated, with per-project documentation migrating to the wiki [1]

* `--attribute 'toc!'` disables AsciiDoc's table of contents [2],
  which Pandoc doesn't like.
* The Pandoc invocation is about as basic as it gets [3].

[1]: https://wiki.gentoo.org/wiki/Gentoo_Wiki:Developer_Central/Project_pages#Migration_from_www.gentoo.org.2Fproj.2F
[2]: http://asciidoc.org/userguide.html#X91
[3]: http://johnmacfarlane.net/pandoc/README.html#using-pandoc