Document LTTng 2.12

Changes from 2.11:

* "What's new?" section.

* Installation section.

* Document the `lttng clear` feature.

* Document the process attribute tracking feature (update the existing
  PID tracking section).

* Add the "inclusion set", "process attribute", "tracked process
  attribute", and "untracked process attribute" terms to the glossary.

* Convert all Babeltrace 1 references/examples to Babeltrace 2 ones.

* Fix English style issues.

* Convert a few simple lists to description lists where appropriate.

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