CONTRIBUTING.adoc: follow EfficiOS AsciiDoc conventions

CONTRIBUTING.adoc: follow EfficiOS AsciiDoc conventions

This patch:

* Removes the version, only keeping the date.

* Sets the `bt2` attribute with a non-breaking space to avoid
  redundancy.

* Keeps only one empty line above a heading.

* Do not repeat full heading title in internal links.

This is in line with the style of other EfficiOS AsciiDoc documents.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: If991b9ac6161a436911f4dd3bc3e80f2eea3fd78
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4422