doc: Move admin-guide to markdown format.

Editing markdown is probably more easier than xml.

pandoc can then be used for conversion to html, pdf and any other necessary
formats. Note that pandoc has the following input and output formats:

Input: markdown and (subsets of) Textile, reStructuredText, HTML, LaTeX, and
DocBook XML.

Output:plain text, markdown, reStructuredText, XHTML,  HTML  5,  LaTeX
(including beamer slide shows), ConTeXt, RTF, DocBook XML, OpenDocument XML,
ODT, Word docx, GNU Texinfo, MediaWiki markup, EPUB, Textile, groff man pages,
Emacs Org-Mode, AsciiDoc, and Slidy, Slideous, DZSlides, or S5 HTML slide shows.
It can also  pro‐ duce PDF output on systems where LaTeX is installed.

All documentation changes can be submitted as changes to markdown and we can
attempt a periodic documentation refresh on gluster.org.

Change-Id: I5dcf7f79184cd6b6d62ce7065d2faa352622f6ac
Reviewed-on: http://review.gluster.org/5232
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>