Convert documentation to AsciiDoc

Convert documentation to AsciiDoc

This huge patch converts the LTTng Documentation sources to the
AsciiDoc format. A few advantages of this move are:

* The AsciiDoc language is much more complete and semantically
  correct than Markdown (or even kramdown). Labeled lists,
  source file inclusion, admonition blocks, custom block and
  inline elements, and complex table layouts are just a few
  of the many features that Markdown lacks and which are needed
  for a high quality technical documentation. The previous
  solution to have those features was to embed HTML elements in
  the Markdown source files, which is a bad idea for an
  appropriate separation of presentation and content.
* The asciidoc(1) tool can convert an AsciiDoc input file to
  a DocBook XML file, making it possible to render the LTTng
  Documentation in any desired format thanks to XSLT and/or
  custom scripts.
* The asciidoc(1) tool can also convert an AsciiDoc input file
  to an HTML5 file, so that the LTTng Documentation can be
  previewed by anyone with tools that are available for all
  the major distributions.

The actual content of the documentation is also completely altered
here. A major overhaul was necessary after two years of untouched
content. The text of v2.7 now follows the Microsoft Manual of Style
(4th Edition), as it is clearly stated in the new
CONTRIBUTING.adoc file. Various improvements are introduced to this
documentation, including:

* Diagrams are redesigned and updated, and new diagrams are
  added.
* The "What's new" section presents the new features as categorized
  lists instead of flat paragraphs.
* The "What is tracing?" section was proofread by a professional.
* LTTng alternatives are added.
* More tables are used.
* New LTTng availability in major distributions.
* Clear examples.
* New glossary section.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>