Update `CONTRIBUTING.adoc`

Update `CONTRIBUTING.adoc`

Changes:

* Change to Asciidoctor format. This is what GitHub uses, so this guide
  is correctly converted on GitHub.

* Update reference count section's terminology and function names.

* Update logging section:

  * Reorganize sections, making guides explicit.

  * Header paths.

  * More details about the available log levels.

  * Library-specific logging statement macros (more or less copied from
    `src/lib/logging.h`).

  * Component-specific logging statement macros.

  * Have clear guides on how to instrument generic, library, and
    component class C source and header file

  * More details about which log level to choose.

  * More details about which logging tag to choose.

Removing the `CONTRIBUTING.html` global Makefile target as we cannot
use Asciidoc to build `CONTRIBUTING.adoc` anymore.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I98effa1fb087274c453014d95115d471b5b115ad
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1446
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>